The telephone installation at the premises of a subscriber may range from a single telephone instrument to a large private branch exchange having upwards of 1,000 extension stations and more than 100 lines to the public exchange. This chapter describes the apparatus required for use on installations ranging from a single instrument connected to the exchange to two or more instruments arranged for extension working. Details in regard to private manual branch exchanges (P.M.B.X.) will be found in the chapters describing the separate telephone systems, whilst private automatiCentral Batteryanch exchanges are dealt with in Volume II. The following definitions refer to the terms used in this and subsequent chapters, and they are taken from the British Engineering Standards Association's Publication No. 204, 1930:
The necessity for an induction coil was first realised by Thomas A. Edison and may be shown by considering the conditions which exist when a transmitter of, say, 10 ohms resistance when quiescent is joined in circuit with a battery, a receiver, and a line circuit, the whole having a total resistance of 1,000 ohms. If, now, a particular sound wave produces a variation of 1 ohm in the resistance of the transmitter, it will be evident that the variation in the total resistance of the circuit is 1 ohm in 1000 that is to say, a percentage value of one-tenth of per cent. But the variation in the resistance of the transmitter considered separately amounts to 1 ohm in 10 that is, a variation of 10 per cent. Thus the variation in the resistance of the transmitter itself has been swamped by the large constant resistance of the remainder of the circuit.
The value of the received speech could, however, be increased by a very large increase in the battery power applied to the transmitter, the limit only being reached when the critical value of the voltage across the transmitter is exceeded.
A telephone induction coil consists of a bundle of fine iron wires around which is wound one or more layers of relatively thick wire, termed the primary winding, and over this a much larger number of turns of thinner wire, known as the secondary winding. The transmitter, the battery, and the primary of the induction coil are connected as shown in (diagram) 160. The secondary is connected to the line with the receiver interposed. The resistance of the primary circuit is kept as low as possible in order that variations in the resistance of the transmitter may produce the maximum possible variation in the resistance of the primary circuit as a whole.
If the resistance of the primary winding of the induction coil is 1 ohm , and the resistance of the two cells included in the circuit is 2 ohms, then, with a transmitter of 10-ohms resistance, a variation of 1 ohm in the transmitter resistance produces a variation of 1 ohm compared with 1 ohm in 1,000 in the instance quoted above. In local battery instruments, it has been found by experiment that the best results are obtained by induction coil No. 12, which has a resistance of 1 ohm with 430 turns of No. 23 S.W.G. single silk-covered copper wire for the primary winding and a resistance of 25 ohms with 1,350 turns of No. 32 S.W.G. for the secondary. The ratio of the number of turns in the primary to those in the secondary is approximately 1:3. The use of a bundle of fine iron wires instead of a solid core is dictated by the necessity for reducing the eddy current loss in the core to as low a value as possible. Were the core solid, the currents induced in it would be a maximum owing to the low resistance of the circulating path through the core; by breaking the core into a number of fine strips, offering a high resistance in the path of the eddy currents, the magnitude of the induced current is reduced; and, as a result, the eddy current loss, which represents a proportion of the total electrical energy, is reduced.
The core is composed of No. 24 S.W.G. well-annealed charcoal iron, left black from the annealing process; this black covering serves as a partial insulation, and so reduces the value of any current induced in the core. In order that the induction coil shall be efficient, it is essential that the changes in the current shall produce corresponding changes in magnetisation and, to assist in effecting this, the coil has an open magnetic circuit. A transformer, with a closed magnetic circuit, is more efficient in the transmission of speech currents; if it were used in place of the induction coil, however, the permanent flux due to the continuous current would tend to saturate the iron core, thereby rendering the instrument useless for the transformation of speech currents. Further, the increased hysteresis and eddy current losses due to the greater volume of iron would cause a reduction in the power efficiency of the transformer, and practically treble the impedance of the primary and secondary windings.
With Central Battery telephones this increase in impedance would not be of much consequence, but with L.B. instruments the secondary winding is an impedance in series with the receiver, and it is necessary that this impedance should be as low as possible. Finally, assuming that a satisfactory transformer could be manufactured, its cost would be greater than that of an induction coil. Hence, the induction coil with an open magnetic circuit is employed in telephone instruments. The connections required when an induction coil is employed are shown in (diagram) 160. The transmitter speaking battery, and primary of the induction coil are connected in series, and the secondary of the induction coil and the receiver are connected to line. Variations in the resistance of the transmitter produce variations of current in the primary circuit, and these variations result in the induction of alternating current in the secondary winding of the induction coil.
Signalling arrangements must be added to the speaking circuit shown in (diagram) 160 to provide a calling signal. A trembler bell, a battery, and a key at each end of the circuit, connected as in (diagram) 161, if substituted for the telephone apparatus by means of the switch, are adequate for this purpose. Depression of the ringing key at station L connects the battery to the lines, thus causing the bell at station M to ring. Similarly, station L may be called by depressing the key at M.
The principle of the trembler bell is illustrated in (diagram) 162. It consists of an electromagnet, the circuit of which is completed through the armature. This armature and the striker are carried by a flexible steel spring to which is attached a light contact spring normally resting upon the contact screw. The passage of a current through the coils causes the armature to be attracted and the hammer strikes the bell gong. The circuit is broken at the contact spring, the armature is released and flies back; the circuit is closed again and the electromagnet energises once more. This process is repeated indefinitely while current flows through the coils. The trembler bell used by the Post Office (Bell 56A and 56B) has two windings, each of 50 ohm resistance, connected to four connection plates, whereby the coils may be joined in parallel (25 ohms) or in series (100 ohms). Bell 56D has coils of 50 ohms resistance shunted by a resistance of 1,200ð ohms
At the moment when the circuit of an electromagnet is broken, its magnetic field collapses, thereby causing an E.M.F. of considerable value to be induced in the coil. This self-induced E.M.F. is in the same direction and adds to the E.M.F. of the battery, and produces a spark between the contact points when the connection is broken. In order that the sparking may not seriously impair the contact, both the contact spring and the contact screw are tipped with a hard metal which will not oxidise, and is not fusible save at very high temperature. Platinum fulfils this condition, but owing to its prohibitive cost it is now frequently replaced by an alloy of gold and silver (90 parts silver to 10 parts gold), or one of platinum, gold, and silver (7 parts platinum, 67 parts gold, and 24 parts silver), known as P.G.S. The tendency to cause sparking at the point of disconnection in a circuit having self-induction may be reduced to negligible dimensions by placing a non-inductive high resistance shunt across the inductance, thus providing an alternative path for the cur Tent due to the E.M.F. of self-induction other than that across the point at which the circuit is broken. Another method, considerably used in automatic telephone circuits consists in placing a small condenser in series with a resistance across the point at which the circuit is to be made and broken. The condenser forms a path of low resistance until charged, whilst the function of the resistance coils is to prevent welding of the contacts by the heavy rush of current to charge the condenser, which would flow through the contacts at the instant of closure if the resistance were not added.
In order to combine the speaking circuit on (diagram) 160 and the signalling circuit on (diagram) 161, it is necessary to introduce a switch which will change the one set of connections for the other, and this is accomplished by a gravity switch controlled by the receiver hook. The receiver ((diagram) 163 normally rests on the hook or cradle, and in this position the signalling arrangements on (diagram) 161 are connected; whilst on raising the receiver, so allowing the spring to raise the hook and operate the gravity switch, the speaking circuit is substituted. When the receiver rests upon the hook, the bell is connected to the A and B wires through the lower contact of the gravity switch via the press key. When the receiver is removed, the secondary of the induction coil and the receiver are connected to the lines. In order that the speaking battery may only furnish current when the instrument is in use, the circuit is completed by the insulated spring forming the upper, part of the gravity switch. Thus the removal of the receiver not only connects the secondary and receiver to the lines, but also connects the speaking battery, the transmitter, and the primary of the induction coil in series. To economise battery power, the speaking battery is also used for ringing purposes, being augmented by the additional cells required to produce a ring at the distant end. When two receivers are required, these are invariably placed in parallel (the reason being that in series their inductance is four times as great as when they are in parallel). The current from the A wire passes through the secondary winding of the induction coil through the two receivers in parallel and back to the B wire. When a microtelephone (vide page 296) is used, the connections are somewhat simplified, since a contact lever is provided to connect the battery, thus dispensing with the two contacts shown in (diagram) 163, the act of grasping the handle serving to complete the circuit of the transmitter ((diagram) 164).
On lines within the same building, the battery-ringing telephone is generally adopted, owing to its cheapness. On long lines it is necessary to introduce a relay to economise battery power, and in such circumstances the magneto-ringing telephone is generally preferred. The fundamental connections of such an instrument are illustrated in (diagram) 165. It will be seen that the transmitter, the primary of the induction coil, and the speaking battery form a local circuit. This circuit may be closed either by a gravity switch (as illustrated) or a microtelephone press key. With the telephone resting, the magneto bell in series with a condenser (see page 2597) is connected across the lines via the contacts of a switch forming part of the magneto generator and a second switch forming a portion of the gravity switch. When the handle of the magneto generator is turned, the bell is disconnected and the generator is connected across the A and B wires. When the receiver or microtelephone is removed from its rest, the local speaking circuit is closed, the magneto bell disconnected from the line, and the receiver in series with the secondary winding of the induction coil is connected across the A and B wires. The method in which these circuit changes are effected in the types of telephone instrument used in a magneto system varies considerably, but the principle remains the same.
The magneto bell has remained substantially the same in principle, since it was introduced by Dr. Watson. It is designed to ring with alternating currents furnished by the generator in return for the mechanical energy expended in turning the crank handle. It consists of an electromagnet BA, with a pivoted armature JJ, rendered magnetiCentral Battery induction from a permanent magnet NS ((diagram) 166). A thin steel rod carrying a brass ball K is fixed to this armature JJ at right angles to it, and placed between the two bell domes M and L. The south pole of the permanent magnet is placed below the centre of the armature, thus rendering the centre, north, and both ends of south polarity. A current flowing through the electromagnet in one direction causes A to become north and B to become south at their lower ends. The armature is, therefore, repelled at B and attracted at A, thus causing the hammer to strike the left dome. Reversal of the current makes A south and B north, hence the armature is attracted at B and repelled at A, and the hammer passes over and strikes the right dome. The alternating current from the generator causes the hammer to vibrate between the two domes. The play of the armature is adjusted by varying the distance between the armature and the coil cores by means of the nuts on the screwed pillars F, E, of the framework. The Post Office standard magneto bell (Bell No. 1A), illustrated in (diagram) 167, has a much longer electromagnet, and is wound with a larger number of turns of thicker wire, thus giving greater sensitivity and higher inductance.
When placed in shunt across a line, the greater its inductance, the less is its effect in shunting speaking currents. The play of the armature is regulated in the same manner as the bell in (diagram) 166, and the oxidised bell domes are adjusted to the hammer, the domes being bored and fixed eccentrically on the pillars. Magneto bells are now almost invariably wound to a resistance of 1,000 ohms. A steady ring should be produced by a good generator through 30,000 ohms connected in series or with the bell shunted by a non-inductive resistance of 50 ohms. The same bell is used on the Central Battery Signalling No. 1 system, but the coils are joined in parallel so as to give a resistance of 250 ohms. There are, however, some old installations in which a special 100 ohm bell is still in use.
In an early pattern of the Ericsson skeleton instrument ((diagram) 195) the bell (Bell N.T. 1,003) consisted of a permanent magnet armature pivoted on the yoke of the electromagnet and playing between the pole pieces, the armature being extended by a small brass rod carrying the bell hammer. This gave place to a bell (Bell N.T. 1,003A) with an induced magnet armature placed in front of, instead of over, the extended pole pieces. This change of type was necessary owing to the gradual but certain demagnetisation of the permanent magnet armature by the passage of ringing currents, thus causing unreliability and failures.
Another modification of the magneto bell (Bell 9A) is used on the wall telephone shown in (diagram) 186. The coils are somewhat longer, and the single permanent magnet is replaced by two smaller magnets passing from each end of the yoke to a position over each pole piece. The armature pivoted over the pole-pieces carries the striker, which is bent so that the hammer plays between the vertical bell domes. The only advantage gained by this design is that the arrangement is flatter.
Magneto signalling could be arranged by connecting a generator and magneto bell, in series, in place of the trembler bell shown in (diagram) 161, the ringing key and the battery being removed. If this were done, the alternating current from the generator at station L would encounter the impedance of the generator and magneto bell at L, in addition to that of the generator and bell at M. In very old instruments the generator and bell were placed in series, but an automatic switch, termed a cut-out, was added to the generator, and was arranged to short-circuit the armature until the handle of the generator was turned; this arrangement cut out the impedance of the armature at a station when a ring was being received from the other station. A further disadvantage of the series arrangement of generator and bell was that the bell at the calling station rang whenever a call was made. This gave rise to confusion when an extension bell was in use, as there was no means of ascertaining whether the ring was due to an incoming or an outgoing call. Further, the additional noise was objectionable. In all modern magneto telephone instruments, the ringing current passes to the line without negotiating the magneto bell, which is either short-circuited or disconnected when the handle of the generator ((diagram) 168) is turned. This function is performed by the automatic cut-out. The spindle of the generator handle passes through a tube upon which the driving wheel is mounted. Before the driving wheel can be revolved, a transverse pin must ride up in a slot, thus moving the handle and its spindle bodily to the right ((diagram)s. 169 and 170). As the handle is turned from A to B ((diagram) 171), the pin moves from A to B and carries with it the whole shaft (shown in heavy black lines), and this allows the contact spring a to move forward, thus producing the changes required in the contacts between the cut-out springs. The condition required is equivalent to a two way switch, one position joining up the bell and the other the generator.
In a Telephone No. 16 ((diagram) 195), the cut-out springs are arranged as indicated in the diagram of connections ((diagram) 196) and are placed close to the driving wheel. The Post Office standard magneto generator is the Generator No. 4C; it has three horseshoe permanent magnets, and its armature is wound to a resistance of 400-2 -. When circumstances call for a more powerful generator, No. 8A is employed; this generator has four permanent magnets, and its armature has a resistance of 500 ohms.
The actual arrangement of the receiver hook and its associated gravity switch varies with different manufacturers, and many varieties are encountered in practice. The gravity switch consists of a number of springs and contacts disposed in a simple manner, and it will suffice to consider the details of a few typical patterns, since examination of the actual apparatus will, if the principle of the connections required is fully understood, at once reveal the functions of the various parts of any other gravity switch.
(diagram) 174 shows the details of the switch used in the old pattern of Telephone No. 11; the general appearance of this instrument and a schematic diagram of its connections are shown in (diagram)s. 186 and 187. The additional contacts shown diagrammatically on the switch arm close the primary circuit, and the upper front contact connects the secondary circuit when the receiver is removed from the hook. The lower front contact is connected to the magneto bell, the generator armature or the bell being connected to the line by the cut-out springs, according to whether the generator handle is being turned or is at rest, respectively.
The Post Office standard receiver hook and associated gravity switch (Switch Receiver No. 1) is shown in (diagram) 175. The hook is detachable, and this greatly facilitates the packing of the instrument. A slot is cut on the underside of the hook and this is engaged by the front part of the pivoted rocker. A strong spring serves to raise the hook when the receiver is lifted, and an insulated arm operates the gravity switch springs arranged vertically behind it. The two long springs are mechanically coupled by an ebonite pin. The springs here shown are as used on a Telephone No. 4, but the number of springs can be varied to meet the requirements of any particular case. For Central Battery working, the receiver hook and gravity switch are arranged as shown in (diagram)s. 176 and 177.
The actual arrangement of the receiver hook and its associated gravity switch varies with different manufacturers, and many varieties are encountered in practice. The gravity switch consists of a number of springs and contacts disposed in a simple manner, and it will suffice to consider the details of a few typical patterns, since examination of the actual apparatus will, if the principle of the connections required is fully understood, at once reveal the functions of the various parts of any other gravity switch.
(diagram) 174 shows the details of the switch used in the old pattern of Telephone No. 11; the general appearance of this instrument and a schematic diagram of its connections are shown in (diagram)s. 186 and 187. The additional contacts shown diagrammatically on the switch arm close the primary circuit, and the upper front contact connects the secondary circuit when the receiver is removed from the hook. The lower front contact is connected to the magneto bell, the generator armature or the bell being connected to the line by the cut-out springs, according to whether the generator handle is being turned or is at rest, respectively.
The Post Office standard receiver hook and associated gravity switch (Switch Receiver No. 1) is shown in (diagram) 175. The hook is detachable, and this greatly facilitates the packing of the instrument. A slot is cut on the underside of the hook and this is engaged by the front part of the pivoted rocker. A strong spring serves to raise the hook when the receiver is lifted, and an insulated arm operates the gravity switch springs arranged vertically behind it. The two long springs are mechanically coupled by an ebonite pin. The springs here shown are as used on a Telephone No. 4, but the number of springs can be varied to meet the requirements of any particular case. For Central Battery working, the receiver hook and gravity switch are arranged as shown in (diagram)s. 176 and 177.
It may be added that the great extension of overhead power circuits has rendered it necessary that none of the exposed metal work of a telephone should be in direct electrical contact with the line, since the possibility of contacts between power circuits and telephone circuits cannot be ignored.
The use of a microtelephone simplifies the form of switch required, since the circuit of the transmitter is completed by the press key. The instrument illustrated in (diagram) 190 (Telephone No. 9) is, however, provided with extra contacts so that an old type of microtelephone which has not a press key can be used if desired. The arrangements will be apparent from a consideration of (diagram) ??
Flexible connections are extensively employed in telephony, but every effort is made to avoid them whenever possible, as they constitute the weakest link in the chain of telephone engineering. The cords now used are the result of endless experiments, and are probably as reliable as any flexible connection can be. The conductor consists of copper, copper alloy, or gilt copper ribbon (10 mils wide by 1 mil thick) spiralled upon a silk thread (5 to 7 mils in diameter), with from 50 to 60 turns per inch, the finished conductor having a diameter of 50 mils and a resistance of 0.24 ohms per yard. A silk braiding followed by a green mercerised cotton outer braiding complete the cord.
A dissected two-way cord (Cord Instrument No. 222) used for connecting up a receiver is shown in (diagram) 178; the two conductors are twisted together and a strain cord is bound in, so that the points at which the cord is connected to the terminals may not be subjected to any appreciable stresses.
The plaited four-way cord required to join up a local battery microtelephone (Cord Instrument No. 421) is illustrated in (diagram) 179. The ebonite button serves to secure the cord to the plug ((diagram) 182) and to prevent stress on the connections. The other end of the cord has the length of its termination so arranged that the red and green conductors reach to the receiver, the blue one to the press button, and the white conductor to the transmitter collett.
The cord illustrated in (diagram) 180 is similar to that shown in (diagram) 179, but is terminated differently. It is used to join up a connection strip to a table telephone.
The cord shown in (diagram) 181 is that used for connecting the Central Battery hand microtelephone (Telephone No. 164) to the connection strip in the pedestal of the Telephone No. 162.
The plaited type of cord used on table telephones is found to be less prone to kinking than cords of the round or solid pattern. For operators' telephones, a protective leather sleeve is added at the junction of the plug and cord. On rifle ranges, street call offices, and in fish shops, a special waterproof cord is usually employed; it has been noticed that in street call offices the ordinary cord is liable to become saturated as the result of contact with overcoats and mackintoshes during wet weather.
Where a waterproof cord is required, the conductor is single, close lapped with 18 ends of 60/1 soft cotton, followed by a single lapping of pure India-rubber ribbon, stretched, and applied so that its edges overlap. A close lapping of 20 ends of 60/1 soft cotton is next applied, and the cord is finally braided with mercerised cotton.
The specification for cords provides tests for the materials of which the cords are made, and is rigorously enforced, with the result that the utmost perfection in manufacture is assured.
The connection of a microtelephone on a wall instrument is made with the plug and jack shown in (diagram)s. 182 and 183, thus simplifying the replacement of a faulty microtelephone. This plug was used also to connect up the operator's headset on switchboards, but it has given way to the four„way circular plug shown with its associated jack in (diagram)s. 184 and 185. The four sections of the plug are insulated from one another by ebonite, and engage and make contact with the four springs when inserted in the jack. A plug and jack of this pattern are used in one form of extension working. (See page ???.)
The standard telephone instrument designed for fixing to a wall is the Telephone No. 11. An old pattern of this instrument is shown in (diagram) 186, and a diagram of the connections in (diagram) 187. The modern form of the instrument is illustrated in (diagram) 188 and its connection in (diagram) 189. The telephone has an inset transmitter (No. 3), and is provided with a generator No. 4 and a Bell No. 9A. A form of wall instrument employing a hand microtelephone is the Telephone No. 59, illustrated in (diagram) 190. The connections made by the gravity switch, controlled by the microtelephone cradle, are detailed in (diagram) 191
The standard telephone instrument arranged for standing upon a table is the Telephone No. 4 ((diagram) 192). Since this is simply a transmitter, a receiver, and a gravity switch, the instrument is not complete, and a separate hand generator is provided, usually fixed to the desk or table in close proximity to the table telephone; the induction coil and magneto bell, with the condenser (should one be required), are fitted in a case and are known as a Bell Set. The bell set for use with the telephone is Bell Set No. 5 if the condenser is not necessary, and Bell Set No. 15 if a condenser is required The bell set need not be fitted close to the telephone so long as it is within audible distance of it. The primary battery is placed in a battery box and fitted close to the bell set to which it is connected. A diagram of the connections required to wire up a telephone station, using a Telephone No. 4, a Generator No. 4CN and a Bell Set No. 15, is shown in (diagram) 193, while the corresponding schematic diagram is given in (diagram) 194.
An earlier form of table set is the Ericsson skeleton instrument, Telephone No. 16, illustrated in (diagram) 195, and it is perhaps the prettiest magneto telephone which has ever been designed. It is entirely self, contained, except for the local battery, which is accommodated in a battery box. The permanent magnets of the generator form the support upon which the instrument rests, and the magneto bell is of the type described on page 280. The induction coil is placed above the magneto generator, and the microtelephone is carried on a cradle which actuates the gravity switch. The connections of the instrument are shown in (diagram) 196, and it is connected up by means of a flexible cord terminating in a connection strip which is fixed at a convenient point on a wall or table.
In the diagram shown in (diagram) 165, the circuit of the magneto bell includes a condenser (capacitor) of 2 F capacitance. Other diagrams shown do not include the condenser. Without the condenser the A and B wires are looped, whether the receiver or microtelephone is on the rest or not. With the spread of automatic telephone systems it has become necessary that magneto telephones shall be arranged to provide through clearing facilities to such exchanges when they are connected under certain conditions on junction calls. To enable this to be done, it is necessary that the condition of the subscriber's telephone when the receiver is on its rest shall be that the lines are disconnected. This is accomplished by adding a condenser in series with the magneto bell; the circuit is now open to receive alternating currents, but direct current cannot flow until the receiver is removed from its rest. A magneto system providing these facilities is said to be "condensered." Any of the instruments shown without the condenser can be condensered simply by adding a 2 F condenser in series with the magneto bell.
In this system a local battery supplies the current for the transmitter, whilst the current required for signalling is derived from a central battery located at the exchange. The signalling conditions in the No. 1 system are that the A-wire is connected to earth through the gravity switch and magneto bell ((diagram) 197) when the receiver is on its rest, whilst with the receiver removed the A and B wires are looped. The calling signal is thus given by looping the lines and the clearing signal by earthing the A-wire.
The standard wall set (Telephone No. 3) is similar to the Telephone No. 11 ((diagram) 198), but the generator is omitted and the internal connections modified, so that when the receiver is on the rest the earthed magneto bell is connected to the A-wire. The bell coils are connected in parallel, so as to offer a resistance of 250 ohms only for signalling purposes.
The standard table telephone consists of a Telephone No. 4 and a Bell Set No. 5 which contains the induction coil and 250 ohm magneto bell (500 ohm coils in parallel). The bell set is fitted at some convenient point adjacent to the position of the table telephone, and is joined up by a length of 4-conductor flexible cord, as shown in (diagram) 198.
An earlier pattern of table set is the Telephone No. 26. This instrument is entirely self contained, except for the local battery, and is illustrated in (diagram) 199. The magneto bell is of the single dome pattern and its coils are wound to a resistance of 100 ohms. (diagram) 200 shows the detail of the cradle and gravity switch; the cradle rests upon a spring and the adjustment of this spring is somewhat difficult, a good deal of care in bending being necessary to ensure reliability in action.
In this system the method of signalling is by means of a loop when the receiver is removed from its rest ((diagram) 201) which is substituted by a 1,000 ohm magneto bell in series with a 2 F condenser when the receiver is replaced. The system is thus the same as the Central Battery system so far as the signalling arrangements are concerned. The transmitter current is, however, supplied from a local primary battery as in the Magneto and Central Battery Signalling No. 1 systems.
The standard wall telephone is the Telephone No. 69, which is similar to the Telephone No. 59, except that the generator is omitted.
The standard table telephone is a Telephone No. 4 together with a Bell Set No. 15, and the combination is connected as shown in (diagram) 202. The schematic diagram of this combination is shown in (diagram) 203.
The standard wall telephone is Telephone No. 121 Central Battery, illustrated in (diagram) 204. With the spread of the automatic system since 1912, when the first automatic exchange in Britain was installed at Epson, it is desirable that the telephones used on automatic and Central Battery manual systems should be interchangeable, and this telephone provides this facility. Behind the notice frame is an aperture for accommodating a dial which is required when the instrument is to be used on an automatic system, and the internal connections of the instrument are so arranged that the dial may be readily connected. The wiring of the telephone is shown in (diagram) 205, the terminals marked l, 2, 3, 4, and 5 being those required for the dial connections.
(diagram) 206 is a schematic diagram of the connections and applies to the majority of Central Battery telephones, whether wall or table types; the action of the induction coil connected across the A and B wires, whilst, when the receiver is removed, the wires are looped through the primary winding of the induction coil and the transmitter. The impulsing springs of the dial occupy the position marked X in the diagram when the telephone is used on an automatic system.
An earlier form of wall set is the Telephone No. 101; it is similar in appearance to the Telephone No. 121 Central Battery, except that no facility is provided for the addition of a dial. The connections of the instrument are shown in (diagram) 207. A still earlier form of wall set is the Telephone No. 1 ((diagram) 208), its connections being as shown in (diagram) 206.
The standard table telephone is the Telephone No. 150 Central Battery ((diagram) 209). This instrument is incomplete, and the induction coil, magneto bell, and 2 F condenser are provided by a Bell Set No. 1. The connections of the complete installation are shown in (diagram) 210. The base of the telephone contains space for a dial, required for use when the telephone is connected to an automatic system; when used on a Central Battery system, this space is covered by a dummy dial which accommodates the instruction label.
An earlier form of table set is the Telephone No. 2, illustrated in (diagram) 211. This telephone is used with a bell set, and the connections ((diagram) 212) are similar to those of the Telephone No. 150 except that no facility for the addition of a dial is provided.
In all the foregoing types of telephone, the transmitter is the No. 1 transmitter described on page 233. The receiver hook and associated gravity switch are illustrated in (diagram)s. 176 and 177. The induction coil is the No. 14 type (see page 274), and the magneto bell has a resistance of 1,000 ohms; the condenser has a capacitance of 2 F.
Central battery systems were less tolerant of the incidental variations in resistance of telephone transmitters which occurred when they were moved, and fixed„transmitter telephones were introduced to replace early handset telephones with the changeover to Central Battery and automatic working. In about 1924 Messrs. Siemens Brothers of Woolwich, in conjunction with the Post Office, commenced research to overcome the defects in existing telephone handset transmitters. This research culminated in the production of the Transmitter No. 10. To accommodate the new transmitter, an all moulded telephone was designed - the "Neophone". It was originally moulded in black bakelite but, shortly after its production, urea„ formaldehyde plastics became available, and it was moulded in that material. Using pigmentation, the telephone was produced in red, green, ivory, and mottled brown. The Neophone had no bell within its case and was originally used with a wall mounted bell set. Later, bell cases were manufactured from the same materials as the telephones and could be fitted as a plinth to the base of the telephone.
The Neophone ((diagram) 213) was the first telephone to be made entirely in plastic. It was adopted by the Post Office as one of its standard models and was also used by several overseas administrations. Variations of the telephone were also made by other companies.
The receiver and new transmitter are assembled together in a moulded bakelite handle to form a microtelephone (Telephone No. 164). In the position of rest, the weight of the microtelephone presses down an horizontal bar extending between the two arms of the cradle; this causes the operation of the gravity switch accommodated in the base of the instrument. As with the standard Central Battery table set, this telephone is designed to be interchangeable with similar instruments used on automatic systems, and the opening for the dial is covered by a dummy dial mounting which serves to hold the label and instruction card.
The telephone uses an inset transmitter, the No. 10 type, described on page 237, being employed. The receiver is contained in an aluminium case, and the permanent magnet is a straight bar of Central Batterylt steel. The transmitter case is provided with a perforated metal cover, the holes in this cover being out of line with those in the base of the mouthpiece to prevent any object being pushed on to the diaphragm. The plane of the opening of the mouthpiece makes an angle of approximately 45 degrees with the line of the handle.
The connections of the instrument, which is used in conjunction with a Bell Set No. 1, are shown in (diagram) 214. The efficiency of the instrument is so high as compared with former instruments of the microtelephone type that it is necessary to provide means whereby the amount of the sidetone in the receiver, and the tendency of the instrument to "howl" by interaction between the receiver and the transmitter, are reduced. This sidetone is also undesirable in that it causes the telephone user to lower his voice in an unnatural way. Campbell in 1920 announced his invention of the A.S.T.I.C. - Anti Side Tone Induction oil. This is effected by the small transformer (Transformer No. 35A) mounted in the base of the instrument. One coil of the transformer is connected across the transmitter and the other across the receiver, and, in addition, the two coils are connected together on the "auto-transformer" principle. The connections are such that the normal output from the transmitter to the receiver is opposed by the subsidiary output due to the transformer, thereby reducing the side tone to comfortable limits.
This microtelephone instrument can also be used on local battery systems.
The transmission efficiency of a local battery telephone connected by a line of high resistance to an exchange is greater than that of a Central Battery telephone. since in the latter case the amount of current supplied to the transmitter is very small on account of the high resistance. Accordingly, it is the practice to employ a local battery instrument when the requisite transmission cannot be given by a Central Battery telephone. The instrument used is a Telephone No. 154 in conjunction with a Bell Set No. 21. The telephone is of the table type; it is fitted on a shelf in lieu of a wall instrument, should this arrangement be preferred. A special three-winding induction coil (Coil Induction No. 15) is used, and is accommodated in the bell set. The primary winding is wound with 400 turns of No. 23 S.W.G. wire to a resistance of about 1 ohm ; the secondary coil has 1,000 turns of No. 32 S.W.G. wire giving a resistance of about 19 ohms, while the tertiary winding consists of 1,500 turns of No. 32 S.W.G. wire to a resistance of about 38 ohms. The connections of the instrument are shown in (diagram) 215, while (diagram) 216 illustrates the same arrangement in schematic form. It will be seen that, when the receiver is removed from the rest, the three coils are connected in series, i.e. on the auto-transformer principle; the primary is in the local circuit of the transmitter, the secondary is connected via the 2 F condenser and the gravity switch to the receiver, while the tertiary, in series with the other two windings, is connected across the lines. In speaking, the fluctuations of current in the ordinary winding cause alternating current to be generated in the whole of the transformer, whence they pass to line. Incoming speech currents pass through the transformer, and the current induced in the secondary winding passes via the condenser, the receiver, and the gravity switch. Incoming ringing currents pass through the condenser and magneto bell, so giving the calling signal. The induction coil has been designed as a result of careful experiments to determine the optimum turns required for each of the windings. The amount of direct current passing through the of the lines will not interfere with the efficiency of the instrument. This telephone can also be used on Central Battery Signalling Nos. 2 and 3 systems and, by interposing a generator between the instrument and the line, on magneto systems.
Previously, a Telephone No. 4 and a Bell Set No. 15 were used for local battery telephones connected to Central Battery exchanges. In these conditions a study of (diagram)s. 202 and 203 will show that the direct current from the central battery flows through the receiver during the whole of the time that the receiver is off the rest. This is a condition which also occurs during the transition ease when a local battery system is being converted to Central Battery working. Since the reversal of the current flowing through the receiver coils results in a magnetic field being set up in opposition to the main magnetic field of the permanent magnet, there will be a considerable loss in efficiency under these conditions. The graph in (diagram) 217 indicates the loss in deCentral Batteryls (see Chapter XIX) for different values of demagnetising current. On the other hand, the effect of the permanent current when it is flowing in a magnetising direction is practically negligible up to a value of 100.
To meet the varying requirements of subscribers desiring extension facilities, the Post Office has standardised various arrangements of apparatus which cater for the majority of cases encountered. These arrangements are known by a Plan No., and, in the majority of cases, they are appliCentral Batterye to all telephone systems. They are shown in (diagram)s. 218 to 228 inclusive; in these (diagram)ures the apparatus shown dotted is only required when the exchange line is connected to a magneto exchange, except in the case of Plan No. 5 (see page 317)
The arrangement of the apparatus is shown in (diagram) 218. This scheme provides from one to six internal extensions connected in parallel with the main instrument. All incoming calls are received at the main, the desired extension being called by pressing the appropriate press button and so ringing the trembler bell at the extension instrument. A call can be originated at any extension instrument without the intervention of the main. The telephones are not secret against one another, i.e. a conversation from one telephone can be overheard at each one of the others, and intercommunication between main and extension telephones is not provides.
Here one internal extension instrument is provided, and facilities for receiving an incoming call at the extension, by operating the switch to connect the extension magneto bell to the exchange line, as well as at the main ((diagram) 219). The extension can originate a call without the intervention of the main. Neither secrecy nor intercommunication is provided between the two telephones.
Here, again, one internal extension is provided, but this time facilities for calling the extension instrument from the main are provided; they take the form of a press button and trembler bell, as in Plan No. 1. Further, the extension is secret against the main, and it is interesting to see how this is effected; the exchange line is taken via the extension apparatus on its way to the main telephone ((diagram) 220). The operation of the switch at the extension instrument disconnects the main and also connects a magneto bell to the exchange line, so that should the switch inadvertently be left in this position, an incoming call would be received on the magneto bell at the extension. Intercommunication between the two telephones is not provided.
This arrangement is very much used in private houses where jack points are located at different positions, and a table telephone, terminated upon a plug so as to render it portable, is carried from one position to another as required ((diagram) 221). For systems other than magneto, a magneto bell is associated with each jack point, a bell being permanently connected to the first jack; while, at the other points, the bell is only connected in circuit when the telephone plug is inserted in the jack associated with that bell. In the magneto system, the portable telephone is already provided with a magneto bell; consequently an incoming call is received on the telephone at whichever jack point it is connected. A magneto bell is, however, permanently connected to the last jack. Thus, the installation provides facilities for the reception of an incoming call on the magneto bell permanently wired, whether or not the telephone is connected at a jack point.
This scheme caters for two internal extensions where communication between the telephones is only occasionally required ((diagram) 222). Incoming calls are received normally at the main instrument; either extension can be called by pressing the appropriate button and turning the handle of the generator. When the circuit is connected to a Magneto, Central Battery Signalling No. 2 or Central Battery Signalling No. 3 exchange, the switch shown dotted replaces the four-position switch, containing the bell, generator, and indicator. The extensions call the main by a generator ring, which is received on the bell associated with the switch. The purpose of the indicator is to show when an extension is using the exchange line. For night service, calls can be received on one or other of the extensions, but not both; a switch is fitted at the selected extension and, when operated, the magneto bell at that extension is connected to the exchange line, the switch at the main instrument being left in the through position.
The lay-out of the apparatus is similar to that for Plan No. 5. Certain modifications to the wiring are made with the object of providing secrecy facilities.
This arrangement provides an internal or external extension with intercommunication between the main and extension telephones ((diagram) 223). The switch shown dotted is used if the exchange line is connected to a Magneto, Central Battery Signalling No. 2 or Central Battery Signalling No. 3 exchange.
This is similar to Plan No. 7, but certain modifications to the wiring are made to provide secrecy.
This scheme provides facilities whereby one extension can be connected to either of two exchange lines at will ((diagram) 224). Either main instrument can call the extension by means of a press button which operates a trembler bell at the extension instrument. A switch for connecting the extension telephone to either exchange line is provided. Secrecy facilities are not provided
This is similar to Plan No. 8, except that secrecy conditions are provided ((diagram) 225). When the switch at the extension is operated, the corresponding exchange line is connected to the extension telephone, the main instrument being disconnected. The scheme does not provide intercommunication between the telephones. This arrangement is only provided when the public exchange is Central Battery or automatic.
This scheme is only appliCentral Batterye to an extension connected to a private branch exchange. Incoming calls are received on extension line No. 1, and the operation of the switch enables an outgoing call to be made over extension line No. 2 for the purposes of obtaining information required for dealing with the incoming call.
This is only appliCentral Batterye to a private branch exchange extension, and provides facilities whereby the extension can be made secret against the P.B.X. when connected to the exchange line ((diagram) 227). Each exchange line is connected through a jack at the extension, and the extension line incoming from the switchboard also terminates upon a jack. The extension telephone terminates upon a plug and, by inserting the plug into the jack of an exchange line, the switchboard is disconnected, and the extension telephone connected direct to the exchange line.
This scheme provides facilities whereby an incoming call can be filtered through the main and extension No. 1 telephones before being passed forward to extension No. 2 ((diagram) 228). The scheme is appliCentral Batterye, for example, to the exchange line of a busy executive, whose private secretary can answer all inquiries on the first extension and only pass calls for ward when satisfied that they are legitimate.
The three-position switch employed is known as an inter-through switch, and there are various forms of switch which will enable the required connections to be made, but it will here suffice to describe the Ericsson pattern (Switch N.T. 14, (diagram) 229). The internal connections of the switch are shown in (diagram) 230. The four springs rest on the inner contacts, and in the positions shown the exchange line is connected through to the extension circuit, the magneto bell being bridged across the lines. When the switch lever is moved over to the right, the upper left spring makes contact with the top contact, whilst at the same time the lower right spring is brought into contact with the bottom contact. In this position the exchange line is connected to the main set instrument, and the magneto bell is connected across the extension line.
If the handle of the cam be moved to the left the bell is placed across the exchange line. In addition to the ordinary function of the magneto bell, an extension of the hammer is so arranged as to release a drop shutter. This shutter is restored to normal by a projection on the switch handle. If the main station has been left unattended, the fact that a call has been received will be obvious from the fact that the shutter is dropped. The connection shown dotted is made when the shutter falls and is required if automatic clearing to the exchange is effected by earthing the centre point of the magneto bell, a system now obsolescent.
It is sometimes necessary to prevent the extension circuit from being connected to the exchange line; for this purpose Switch N.T. 15 is employed, having only two positions. Two extra springs and contacts are added to render it electrically impossible to connect the exchange line to the extension. Similar arrangements to those of Switch N.T. 14 can be made by using three pairs of 5-point jacks (Jack No. 14) and two mechanically-connected three-way circular plugs (Plug 601).
The connections required are shown in (diagram) 231, and it will be seen that the telephones are connected in parallel to the exchange line. All incoming calls are received at the magneto bell at the main instrument, but any one of the stations can call the exchange by lifting the receiver. A call having been received at the main telephone for one of the extensions, the press button at the main set in connection with that extension is pressed, thereby ringing the trembler bell at the extension and indicating that attention is required. For example, the main instrument may be placed in the general office of a firm of solicitors or merchants, and the extensions in the private offices of the partners or managers. Since the magneto bell is normally earthed, it is necessary that it shall be disconnected when any station lifts the receiver, and this is provided by taking the connection through contacts on each of the telephone gravity switches. The extension call bells are actuated by the main set speaking battery, together with the call extension speaking battery, which are connected in series when the relative press button at the main telephone is depressed. The trembler bell and the induction coil are contained in a separate wooden case (Bell Set No. 6) when table telephones (Telephone No. 4) are used.
The four-position switch used in this plan is known as Bell Set No. 7 ((diagram) 232), and the various combinations of connections ((diagram)s. 233 and 234) are determined by the four positions of the pointer of a rotary switch.
Position 1: The main set (Telephone No. 3) is connected to the exchange line and the extension to a magneto bell in the bell set. Thus the main set can call or be called by the exchange, and the extension can call the main station.
Position 2: The main instrument is connected to the extension line, which is called by the generator provided in the bell set. The exchange line is held by the magneto bell placed across the lines (i.e. the switching over of the main set to the extension does not disconnect the exchange line), and thus does not give a clearing signal while the extension is being called by the main station.
Position 3: The main instrument is connected to the extension line and the exchange line to an earthed magneto bell for the receipt of calls from the exchange during the progress of a conversation with the extension.
Position 4. The extension is put through to the exchange, and in order to give a clearing signal to the main set on conclusion of the conversation, a 4,000 ohm eyeball indicator is bridged across the lines in series with a magneto bell. When the extension restores the receiver, thus earthing the A-wire through the magneto bell, a clearing signal is given to the exchange, and on withdrawal of the plug the relay and battery are again joined up. The current flowing through the eyeball indicator is insufficient to operate the line relay (biased against 10 mA), but operates the indicator. The main station cannot overhear the conversation, but if secrecy against the main station is not desired, the main set is placed in bridge by joining the terminals marked A and A1, and B and B1 respectively. The night switch is for the purpose of disconnecting the eyeball signal, and, consequently, the local alarm circuit, when the extension is left connected to the exchange line during periods for which the main instrument is unattended.
The circuit arrangement is similar to that described for Central Battery circuits (see page 331), using Jacks No. 13, Plug No. 407, and Telephone No. 90 or Jacks No. 9, Plug 701, and Telephone No. 4.
The arrangement of the plug and jacks is shown in (diagram) 235. The portable telephone is a Telephone No. 4 connected by means of a Cord Instrument No. 422 to a Plug No. 404. Jacks No. 21 are used at the extension positions and a Bell Set No. 5 is permanently connected to the first position. The magneto bell shown dotted may be omitted at any position from which the magneto bell in the Bell Set No. 5 is audible; when this is done, springs 1 and 4A on the jack are connected together. When the instrument plug is inserted in any jack, the connection between spring 4 and spring 4A is broken, so connecting the magneto bell at that position in circuit, in addition to the magneto bell of the bell set. (diagram) 236 illustrates the connection made when the plug is in a jack at an intermediate or last position.
Central Battery System, Plan No. 1
A Telephone No. 1 is used at the main set, and the extensions, which in (diagram) 237 are table instruments, use the induction coil and condenser of the main set. The bells for gaining the attention of the extensions are joined across the A and B wires, when the press buttons are operated and are rung by the exchange central battery.
If one of the extension telephones is of the wall type, the magneto bell is disconnected and it is connected in parallel across the exchange lines. In this case the extension uses its own induction coil and condenser and does not make use of that at the main set, as is the case of table instruments. The apparatus required is:
It is often desired that the extension station may speak without the possibility of the conversation being overheard at the main set located, say, in the general office. The principle of the arrangement ((diagram) 238) consists in placing both telephones in parallel on the exchange line through a six„point, two-position switch, which, when moved to the secrecy position, cuts off the main instrument. Both instruments are provided with their own induction coil and condenser, and since failure to restore the secrecy switch prevents calls from being received at the main set, the switch in this position joins up the magneto bell in the extension bell set, so that a ring will be received on this instrument. The apparatus required is:
To avoid the use of extension circuits and switches, a table telephone (No. 2) is connected to a Plug 407, and wall sockets are fixed in the various rooms where it is required to use the telephone (e.g. dining-room, study, and bedroom). A Bell Set No. 1 and a socket (Jack No. 13) is fixed in the dining-room, and in each of the other rooms a socket and a magneto bell. These bells are, however, short-circuited until the plug is inserted. Thus, the instrument may be carried to and connected in whichever of the rooms the convenience of the moment dictates. The arrangement will be readily appreciated from a consideration of (diagram) 239. A later arrangement consists in the use of Plugs 404 and Jacks No. 3 and 21.
Central Battery System, Plan No. 7
The arrangements previously described do not provide for intercommunication between the extensions and, where this facility is required, recourse must be had to switchboards. It is, however, possible to provide intercommunication between the main set and one extension by the aid of Bell Set No. 4, The conditions which the arrangement must fulfil are:
These conditions are effected in the four positions of the switch on Bell Set 4, and are shown in skeleton in (diagram)s. 241-244. The Bell Set No. 4 ((diagram) 240) comprises a 4-position switch, indicator 1,000A, generator No. 4, 1,000 ohm magneto bell, 1F condenser, and a 2-cell battery. The main instrument may be a Telephone No. 1, but at the extension set a generator is required for calling the main set, and therefore the apparatus fitted consists of a Telephone No. 1 or No. 2, with Bell Set No. 1, the line circuit passing through the back contacts of a generator No. 4, so that when the generator is used the extension telephone is disconnected. The switch pointer on the bell set normally remains at position 1, and thus a ring from the exchange is received on the bell of the main set. Raising the receiver extinguishes the supervisory lamp at the exchange in the ordinary way. The extension is connected to the extension bell, and thus provision is made at the main set for the receipt of a call from the extension. The extension is called by the generator fitted in the bell set.
In position 2, the positions of the main set telephone and the extension bell, with its condenser, are reversed; but, in addition, a 2-cell battery is placed in the circuit to the extension to furnish speaking current between the main and extension. Calling, in both directions, is by generator. A ring from the exchange is now received on the extension bell.
Position 3 holds the exchange line while speaking to the extension. The necessity for this condition may arise when the main set receives an inquiry which necessitates consultation with the extension before replying. The only change, as compared with Position 2, is that one coil of the bell set magneto bell is joined across the exchange line, thus preventing the lighting of the supervisory lamp at the exchange. Position 4 puts the extension through to the exchange, but inserts an eyeball indicator in the main circuit. At the conclusion of the conversation, when the extension replaces his receiver it disconnects the circuit, and thus the indicator, which has been operated while the call was in progress, falls back to normal. This is known as a negative clearing signal, to distinguish it from a positive clear, which may be defined as a signal which is operated to give the clearing indication. Failure to restore the switch to normal involves the receipt of a subsequent ring from the exchange on the bell set and on the bell at the extension set. On the Bell Set No. 4, links are provided so that the main set may, in Position 4, be connected in parallel across the circuit while the extension is speaking to the exchange, or, by their removal, secrecy of conversation between the extension and the exchange is secured.
This bell set was originally introduced to replace Bell Set No. 4 on Plan No. 5, 5A, 7 and 7A installations connected to automatic exchanges. As it was found to give better transmission conditions than Bell Set No. 4, its use was extended to Central Battery manual circuits. The Bell Set consists of a 4-position switch, an indicator-relay (Relay 257A of 50 ohms resistance), a generator No. 4C, a Bell No. 1A, a 2 F condenser, and a 1 F condenser. A two-cell battery is provided to furnish speaking current on main to extension calls. The connections of the telephones resulting in the four positions of the switch are similar to those of the Bell Set No. 4 (page 332). The internal connections of the Bell Set No. 20 are shown in (diagram) 245, while (diagram) 246 illustrates the connections made in the four positions of the switch.
It will be seen that the indicator relay is shunted by a 2 F condenser instead of by a non-inductive resistance as in the Bell Set No. 4. Also the magneto bell of the main telephone is disconnected by the contact of the indicator relay when the switch is in the through position. By these means, a circuit clear of any bridged apparatus is provided, with the result that, under automatic conditions, there is no distortion of the dialled impulses on account of the introduction of the bell set.
The generator used at the extension instrument is connected in series with a 2 F condenser, this combination being known as Generator No. 4CP. The necessity for the condenser arises from the fact that should the generator handle be turned when the switch is in the through position, the indicator„relay would be operated by the current flowing round the loop provided by the armature of the generator when brought into circuit by the operation of the cut-out spring, whereas it is necessary that this relay should be released in order to connect the magneto bell at the main for the reception of the ring. The indicator-relay is of the pendant armature type, and is provided with a copper sleeve on the core. The armature actuates a single change-over spring set and an indicator attachment, consisting of a balanced aluminium lever which moves an aluminium flag into position behind a rectangular window. The indicator relay was designed by Messrs. Siemens Bros. to meet the following conditions:
In all the telephones illustrated, an extension bell can be connected in series or in parallel with the magneto bell of the telephone by removing a strap between two terminal connection plates in the circuit of the magneto bell and connecting the extension bell thereto; these connection plates are usually marked EB1 and EB2. Loud sounding bells are often required in yards and workshops; in such circumstances an indicator (No. 3501AN) is connected in place of the extension bell, and the local contact of this indicator is included in the circuit of a Bell No. 19A, which has a 6 inch gong and requires a 5-cell battery, or a Bell No. 19A, which has a 12 inch gong and requires an 8-cell battery. When a calling signal is required in situations where the sound of a bell is masked by the noise of running machinery, as in factories and powerhouses, a motor-car hooter is employed; when an electric power supply is available, the electric horn, Buzzer No. 19B, is worked from a battery of five small secondary cells which are kept charged by a current of from 20 to 30 mA supplied from the power mains through suitable resistance lamps. In other circumstances, Buzzer No. 19A is used, and is driven from a battery of two small secondary cells, which are kept charged by a battery of 5 WK1 Leclanché cells connected to them through a resistance of 100 ohms. When the extension bell is not required to ring continuously from the receipt of a call until attention is given at the telephone, a relay 56A (1,900 ohms) is connected in place of the indicator. The extension bell is operated by the relay contacts and, as the relay responds to ringing currents and then restores to normal, the extension bell rings in synchronism with the rings received on the magneto bell at the main telephone. Such extension bells are termed non-continuous ringing bells.
Extensions with intercommunication, other than the Plans already described, necessitate a switchboard, and cordless boards are provided in three sizes, termed switchboards Central Battery Signalling No. 1 or No. 2, or Central Battery, 1+3, 2+4 or 3+9. These boards are arranged to be placed upon a table. Where larger switchboards are required on Central Battery Signalling lines, a Central Battery switchboard is installed and auxiliary apparatus is provided in the exchange lines in order to give the Necessary signalling conditions.
Before considering the electrical circuits it may be stated that the standard arrangements provide that:
The general appearance of a cordless board is shown in (diagram) 415. Description of the electrical circuits will be greatly simplified if the method of operating the board is first made clear by the aid of (diagram) 416. The first two vertical rows each consist of a drop indicator and two keys having three positions, normal (centre) and up or down. The next four vertical rows contain an eyeball indicator and two similar keys associated with the extensions. The two keys on the extreme right relate to the operating telephone; those marked "night" on the left will be described later. Thus the lines and operator's set occupy vertical rows, whilst the three connecting circuits, to which connection is made by throwing the keys, run horizontally. When a key is thrown, it connects its line to the connecting circuit, and by throwing the key of any other circuit on the same horizontal row in the same direction, connection is established between the lines of keys.
This will best be illustrated by following the progress of a typical call.
Exchange line 1 indicator drops, operator moves exchange 1 top key upwards and operator's key upwards, thus connecting her to the exchange via circuit 1.
On ascertaining that extension 3 is required, she throws lower exchange line key to hold exchange and restores upper exchange line key to normal; throws lower extension key to generator while she rings the extension required, throws exchange line top key and extension 2 top key upwards, thus connecting the circuits via circuit 2; and restores the operating key to normal.
Operator's circuit and connecting circuit positions of all keys, and holding position of exchange line keys are locking, while the ringing positions are non-locking.
On the conclusion of the conversation on the exchange lines the eye-ball indicator on the extension line connected is energised and the operator restores the keys to normal.
The smallest board is provided with two connecting circuits, the upper position of the bottom row of keys not being used; the intermediate size has three connecting circuits; whilst the largest size has five connecting circuits, necessitating the provision of a third row of keys.
The circuit connections of the cordless board used for P.M.B.X.s associated with a Central Battery Signalling No. 1 public exchange are shown in (diagram) 417. A relay shunted by a 2 uF condenser is connected in series with the A-wire of each connecting circuit. When an extension key is operated, this relay operated through the A-wire of the extension line and the 250 ohm magneto bell at the extension instrument to earth. The contact of the relay closes the circuit of the eyeball indicator of the extension circuit. When the extension replies, the removal of the earth from the A-wire causes the relay to release, and the eyeball indicator also releases.
In connecting circuits 1 and 3, the operation of the night key disconnects the eyeball indicator from the contact of the series supervisory relay and also disconnects the local contacts of the exchange line indicator from the buzzer. This permits calls to be made form an extension at night-" -the appropriate keys being left in the operated position-" -without the operation of the eyeball indicators in the two connecting circuits.
(diagram) 418 illustrates the connections of the cordless board used at P.M.B.X.s connected to a Central Battery Signalling No. 2 or 3 public exchange. The general principle of switching is the same, the only changes being in the arrangements for signalling. Each connecting circuit contains a repeating coil and condensers; the extension side of this bridge is connected to a double-coil relay which functions as a supervisory relay, the contact of the relay controlling the connection of earthed battery either to the earthed coil of the eyeball indicator or to the A-wire of the exchange line.
With the extension keys normal, earth and earthed battery are connected through the two 1,000 ohm coils of the eyeball indicator and the keys to the A and B wires of the extensions lines. When the receiver at the extension is lifted, the line is looped and the eyeball indicator operates. This constitutes the calling signal.
When an extension key is operated, preparatory to calling the extension, earthed battery through the impedance coil is connected through the contact of the supervisory relay, the night key (if one is provided in the connecting circuit), the auxiliary springs of the operated extension key, one coil of the eyeball indicator to earth. When the extension replies, the double-coil supervisory relay operates, disconnecting the eyeball indicator and extending earthed battery to the A-wire of the exchange line to provide supervisory conditions on the cord circuit at the public exchange. This circuit thus constitutes the supervisory and "flashing" circuit.
The purpose of the 750 ohm retardation coil connected to the moving springs of the lowest exchange line key is to provide a means of holding the exchange line. The night key provides the same facilities as described in connection with the Central Battery Signalling No. 1 type of board.
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