Australian Post Office
The 300 type Telephone


Telecommunication Journal of Australia (Vol. 2, No.2)
OCTOBER 1938

THE 300 TYPE HANDSET TABLE TELEPHONE
T. T. Lowe

Interesting additional facilities and improvements in design are included in the new 300 Type Magneto, C.B. and Automatic Handset Telephones which will be delivered during 1939. Number of Handset Telephones in Service - Recently there has been a marked increase in the demand for Handset Telephones. Fig. 1 indicates the increase in the number of handset and other telephones connected during the past three years and the anticipated development during the next 18 months.

The total number of telephones of all types in service in Australia is about 650,000 and of these, 270,000 are magneto, 320,000 automatic, and 60,000 common battery. The net annual increase in the number of telephones connected is now 36,000, and about 55,000 of the older type telephones in service are being changed annually to the handset type.
The total number of handset telephones installed is about 220,000, 55,000 being magneto, 132,000 automatic and 33,000 common battery. Handset telephones in service are increasing now at the. rate of 72,000 per annum.
By September next year, 180,000 handset telephones of the 566 type and 120,0.00 of the 232 type, making a total of 300,000 instruments, should be in service throughout the Commonwealth. This is exclusive of the 300 type, deliveries of which should commence about the middle of 1939.

Types of Handset Telephones - As the new telephone will be the third type of handset telephone to be used in the Commonwealth, it is necessary to explain the significance of each type and the numbering.

Type 566 - This is the B.P.O. type 162. It is the type already in use in the Commonwealth with anti-sidetone transformer included in the body, i.e., the upper portion of the instrument, and the induction coil, which is not of the anti-sidetone type included in the bell set.

Type 232 - This is the B.P.O. 232 type recently supplied which is similar to  the 162 but with an anti-sidetone induction coil included in the body of the telephone instead of the anti sidetone transformer.

Type 300 - This is the new 13.P.O. 300 type which is described in this article. The distinguishing numbers which ale allotted to these instruments are in the group from 300 to 399.

Handset Telephones Fitted with Anti-sidetone Induction Coils (ASTIC Type) - The 232 and the new 300 type C.B. and Automatic Telephones are the ASTIC type, i.e., fitted with anti-sidetone induction coils. Local battery telephones purchased up to the present are not fitted with anti-sidetone induction coils, but a suitable coil and circuit have been developed, and their use is now under consideration. In view of the importance of the anti-sidetone telephone in its effect upon transmission, it is of interest to discuss the factors which have led to the adoption of this type of instrument.
It is, of course, the ideal of good telephone practice to provide a high standard of transmission efficiency approaching as nearly as possible to direct face to face conversation.
Sending efficiency should be as high as is required to keep the speech level well above the level of induced noise, but not so high as to cause appreciable crosstalk into other telephone circuits. The maximum desirable receiving level is determined, by the limitations imposed by the ear in accepting without strain speech levels above a certain intensity. In a telephone conversation the environment and prevailing noise conditions for the speaker may be quite different to those at the listening end.

In a direct conversation, say across a table, both the speaker and the listener are subject to the same environment including the prevailing noise conditions. The speaker regulates the volume of his voice unconsciously by the ease with which he hears himself under the prevailing noise conditions, and by the ease with which he is hearing the other person and the other person is hearing him. The ease with which the speaker hears himself is the primary controlling factor.

These factors also regulate the volume of telephone speech, but their magnitude and relationship differ from those of ordinary face to face conversation, and vary with different telephone connections. The sidetone of a telephone is substantially higher than the sidetone of direct face to face conversation, and as the speaker is much more accustomed to direct conversation, the extra sidetone of the telephone deceives him by, causing the impression that he is talking louder than he really is.
The three factors which affect conversation, whether direct or by telephone are volume, quality and noise.

The performance of telephones in service is affected by the following factors:-
(a.) The characteristics of the exchange equipment, subscribers lines, junction lines and trunk , lines with which the telephones are associated.
(b) The amount of extraneous noise present at the transmitting and receiving stations.
(c) The effect of sidetone upon the volume speech.
(d) The distance between the user s lips and the transmitter.
(e) The closeness of contact between the receiver and the users ear.

The total noise as distinct from speech in the users telephone ear is made up of the following:-
(a) Leakage of room noise between the  receiver cap and the ear.
(b) Room noise picked up by the transmitter of the local telephone and returned as sidetone to the receiver.
(c) Room noise picked up by the transmitter of the distant telephone, and transmitted over the line to the receiver of the local telephone.
(d) Circuit noise.

An anti-sidetone telephone effectively reduces side-tone. This means that sounds, either noise or speech, acting upon the transmitter are reproduced in the receiver of the same telephone (i.e., the transmitting telephone) at a much lower level, without materially affecting either the sending or the receiving efficiency of the instrument.
In the anti-sidetone telephone the transmitter and receiver are coupled to the line through the anti-sidetone induction coil which in addition to three inductive windings is provided with a balancing network. The circuit which is made up of the following four elements: transmitter, receiver, line and network, when coupled by the transformer (anti-sidetone induction coil) is so arranged that the voltages produced by the transmitter tend to balance out and thus reduce sidetone in the receiver.
In theory the complete elimination of sidetone is possible, but in practice due to the unavoidably wide variations in lice impedance, this cannot be entirely realized. Also variations in the loop resistance affect the transmitter current loss, and, therefore, vary the sending efficiency and sidetone volume.

The ASTIC is arranged to reduce sidetone to the maximum extent on average loop conditions.

Receiving efficiency is reduced by sidetone through reproduction in the ear of the listener of room noise picked up by his own transmitter. In the anti-sidetone telephone the level of room noise in the receiver is lower than the level of received speech resulting in a gain in effective reception when compared with the sidetone instrument. It should be remembered that leakage between the receiver and the ear, if the receiver is not held tightly against the ear, causes a large reduction in volume and permits further room noise to be picked up by the listeners ear.

For either sidetone or anti-sidetone telephones, sidetone is louder than for ordinary face to face conversation, and the effect of sidetone as indicated previously is to make the user think he is speaking louder than he really is. In the anti-sidetone telephone the reduction of sidetone causes the input level to be increased by the speaker with a resultant gain in effective transmission when compared with the sidetone telephone.

A transmitter is really a form of amplifier, and when it is attached with a receiver to a common handle the two components are mechanically and acoustically coupled. A vibration in the receiver may be transmitted via the handle (mechanical coupling) or via the air path (acoustical coupling) to the transmitter which in turn conveys it via the telephone circuit back to the receiver, so renewing the process. Conditions may be set up in which sustained oscillation or howling between the transmitter and receiver will result. If a handset is placed face downwards on, a table, an air column is created which is resonant at a. frequency of about 2500 cycles per second. Acoustical coupling may, therefore, limit the transmission improvement which may be effected under a given set of conditions. The anti-sidetone telephone circuit provides a greater margin of safety against howling
due to mechanical and acoustical coupling.

Handset Telephones
The new 300 Type Handset Telephone which has been developed by Ericsson, England, is designed on the lines of the Swedish Ericsson model and is a complete unit. The 566 and 232 type telephones each consist of two units, a moulded handset telephone and a moulded bell set. The two separate parts of the telephone cars be installed in different locations or combined as one instrument. A magneto telephone requires in addition a separate hand generator mounted in a wooden or moulded case, and two or three dry cells in a battery box. The object of the two unit design was to enable the telephone to be used with the wooden type bell sets formerly associated with pedestal type table telephones in service, thus avoiding the purchase of moulded bell sets. Experience has shown that a combined instrument is now usually required.

It will be interesting to see which type of handset telephone the public will prefer:-
(a) The 566 or 232 type black telephone with a separate wooden cased or moulded bell set and a separate generator when necessary; or
(b) The 566 or 232 type black telephone and a moulded bell set combined as one unit, and a separate generator when required; or
(c.) The 300 type black, ivory, jade green or Chinese red combined instrument.

It is considered that there should be a demand for a 566 or 232 type telephone associated with a wooden bell set as a separate unit for a service where the bell set can be conveniently mounted out of sight, e.g., under the desk or table. Also the 566 or 232 shape may become more popular as a portable telephone than the 300 type due to the ease with which this telephone may be picked up and carried from room to room.

As the 300 Type Telephone will be the Departments standard  instrument, some of the reasons for the change from the 566 and 232 types which ale alike in general appearance are set out briefly :-
(a) As the 566 instrument consists of 10 separate moulded parts, and only 5 are included in the 300 type, the latter design should be more convenient and economical to manufacture as a, combined telephone.
(b) In the 566 type the incidence of cradle breakages and plungers sticking, is high. No cradle is included in the 300 type, and sticking of the brass plunger rods should not occur.
(c) The cradle switch and spring set in the 566 type are not of good design, and trouble has been experienced in service due to unsatisfactory spring contacts. In the 300 type the switch and spring set al-c of greatly improved design. The spring set is fitted with double silver contacts, and adequate spring contact follow is provided.
(d) For certain services it is desirable to replace the magneto bell in the telephone with a trembling bell. This is provided for in the 300 type, but a trembling bell cannot readily be fitted in the 566 telephone.
(e) It is desirable also on certain services for non-locking press button keys to be fitted in the telephones. In the 300 telephone this is provided for, but suitable press buttons for these facilities cannot be fitted readily on the 566 type.
(f) A generator must be provided on magneto services and on C.D. extension services when an extension switch is at the main. In the 300 type the generator is mounted in the moulded case of the instrument, but a separate generator must be provided for the 566 type.

The 300 Type Table Telephone 
The general appearance of the 300 type Automatic and Magneto Handset Table Telephones is indicated in the photograph to the right. In a C.B. telephone the dial is replaced by a dial dummy. It will be seen that the same moulded body is provided on all telephones, the generator in the magneto being accommodated within the telephone body.

The 300 type telephones are designed on modern simple lines and are of pleasing proportions. The ordinary instrument supplied will be black conforming with present practice, but supplies are being obtained also coloured ivory, jade green and Chinese red to meet special cases. All telephones are provided with the Swedish Ericsson type instrument cords which are coloured to match the instruments. Extensive experience in service has indicated that Swedish Ericsson instrument cords are comparatively free from kinking and twisting when in use.

The telephones are provided with new moulded Terminal Blocks No. 20 fitted with moulded covers. Normally the line cord and leading in cable are led in at opposite ends of the terminal block, but both or either may be led in through the bottom by breaking out thin moulded fins. Suitable recesses are provided in the blocks to enable the leading-in cable to be sealed when necessary. To minimize surface leakage raised portions of the moulding are provided between the terminals.

The 300 type Handset Telephones are table instruments and are not designed for wall use. The 566 or 232 type handset telephones fitted with wall brackets will be used as wall instruments.

C.B. and Automatic Handset Table Telephones (Nos. 332 C.B. and 332 A.T.)
The handset and the body of the instrument are supplied in highly polished moulded material, the body being moulded in one piece. All external metal fittings are finished in chromium plate having hard wearing surfaces which are easily cleaned. The metal base plate which carries four rubber feet is readily removed by unscrewing four captive screws, one in each corner, and provides access to the sheet metal chassis on which all the components other than the dial are mounted. The metal base plate is fitted with two metal runners for a directory tray, which will not be supplied. Instead of the directory tray the base plate of each telephone will be provided with a moulded dummy which can be removed by unscrewing two screws from the underneath side of the base plate. A circuit diagram will be provided in. the recessed portion of the upper surface of the metal base plate.

The bell gongs can be adjusted and the instrument cords changed without removing the chassis from the telephone.
The chassis, however, is readily removable when necessary by unscrewing three captive screws. The handset, the magneto bell and instrument cords are interchangeable with similar parts on Telephones 566 and 232. On the upper side of the pressed metal chassis the cradle switch lever, the cradle switch spring set, the dial cord terminal block and the radio interference suppression unit when required are fitted. On the under side of the chassis the magneto bell, the bell gongs, the anti-sidetone induction coil, the dual condenser and the instrument cord terminal block are mounted.

Magneto Bell
The magneto bell polarizing magnet is a cobalt steel rod having an armature pivoted at one end, whilst the other end fits into an iron yoke, slotted to form an effective clamp, for the magnet. If adjustment is necessary, the clamp is slackened off by loosening one screw only, and: the magnet is then free to move in or out of the yoke. Each bell coil is wound to 500 ohms resistance and the coils may be readily connected, either in series or in parallel. On the telephones supplied they are connected in series, the resistance of the bell being 1000 ohms. The bell gongs (Nos. 2 and 2A) are manufactured from different thicknesses of metal which give to each gong a different tone when struck by the hammer, thus Providing a distinctive and pleasing ring. The bell gongs are adjusted in the usual manner, the centre mounting hole being slightly eccentric to the outer rim of the gong; the gongs are turned to vary the spacing between the outer rim and the striker. The gongs are fixed by screws to short projections on the chassis, and to prevent them from working loose, spring steel locking washers are fitted between each gong and the chassis.

Anti-sidetone Induction Coil No. 22
The ASTIC No. 22 is provided with an open type core and has six windings, three inductive and three non-inductive, wound on a moulded bobbin. Fig 6 shows the direction, of windings and the connections. The operation of the anti-sidetone induction coil is referred to in detail later.

Dual Condenser
The two condensers (2MF and 0.1 MF) are accommodated in one metal case, the outer dimension. of which is 1 in. x 1 in. x 3 in. The electrical characteristics of each condenser are similar to those of the ordinary larger size type. The reduction in size is made possible by improved manufacturing processes, and by the use of synthetic wax having a higher permittivity (specific inductive capacity) than paraffin wax. The 0.1 mf condenser is connected across the transmitter to prevent radio interference due to the pickup of radio programmes by the receiver as a result of rectification by the carbon granules of the transmitter.

Cord Connection Blocks 
The two cord connection blocks are of moulded material. The terminals which are fitted with 4 B.A. screws and cupped washers are retained in position: by a half turn on the soldering tag portion.

The cord connection: block for the handset and line cords has 13 terminals, some of which are commoned with metal strips used for altering the circuit arrangements for various “Plan Numbers Services” as will be described in a further article. The terminals marked 4, 5 and 6 are used for the handset cord, the line cord being connected to terminals 1, 2 and 9.

Cradle Switch Mechanism
The cradle switch mechanism consists of a spring controlled pivoted lever which carries the usual polished ebonite plunger tip and contact springs fitted with double silver contacts. The centre spacing insulator of the spring set is much shorter than that usually provided and the short inner springs are tensioned outward ensuring adequate contact follow.
The cradle switch level is operated by two metal plunger rods which project through the cradle portion of the moulded body of the instrument. The plunger rods, which are operated when the handset is replaced in the cradle position, move freely in metal bushes fitted in the moulded case. With this arrangement the liability of trouble due to plunger sticking is reduced to a minimum. The spring set can be adjusted with the chassis removed froth the telephone case and where the chassis is replaced the adjustment will be maintained.

Dial and Dial Dummy
For C.B. operation the usual dial dummy is fitted in the dial aperture and terminals 4 and 5 of the moulded dial cord connection block are strapped. For automatic working a Dial No. 10 provided with a stainless steel finger plate is fitted and connected by means of a five-way instrument cord to the -dial cord connection block. At one end of the dial connection block a spring clip is fitted to keep the dial cord clear of the dial impulse springs.

Radio Interference Suppressor
A radio interference suppressor to absorb the high frequency components of the dialling impulses is, connected when. required to the appropriate terminals on the dial connection block. The unit consists of a capacity and inductance mounted on a moulded base.

Handset Telephones
The handset telephone is of the modern moulded type fitted with a three conductor cord 3 ft. 6 ins, long. The connections from the transmitter recess in the handset to the receiver recess are embedded in the moulded handle and consist of heavy gauge insulated copper wires soldered to screwed brass terminal inserts. The receiver is of the inset type leaving a cobalt-chrome steel polarising magnet, the fixing screws forming the electrical connections. The moulded earpiece screws to the receiver case and clamps the diaphragm in position. The connections to the transmitter inset (B.P.O. No. 13 type) are made by the centre terminal spring, and a double contact fiat spring. The mouthpiece is fixed by a clip operated by a special key to prevent interference to the inset.

Magneto Handset Table Telephone (No. 333MT)
In general appearance the new model Magneto Handset Table Telephone is the same as the C.B. and. Automatic type except that a small hand generator is mounted in the dial Space in the moulded body of the instrument.
The telephone is a complete magneto unit except for a battery box. The first deliveries will be the sidetone type, but future supplies will probably be the anti-sidetone type.

Base Plate
The sheet metal base plate which carries all the electrical components of the telephone is provided on its under side with four rubber feet and two metal cover plates. One cover plate fits over a recess in which is mounted a moulded connection block for the instrument cords. The other cover plate fits over an aperture in the base plate provided to enable the generator cut-out springs to be readily adjusted.

Instrument cords can be changed and the generator cut-out springs adjusted without removing the metal base plate from the telephone. To remove the metal base plate from the telephone the generator handle is first removed by unscrewing the centre holding screw in the generator driving axle. Then the four corner and two side captive screws are unscrewed, and the base plate carrying the components is readily removed.

Induction, Coil (Sidetone Type)
The 1 ohm plus 19 ohm induction coil which is of the open core type fitted with a moulded bobbin is mounted on the upper side of the base plate near the front. The primary winding of the induction coil has 400 turns and the secondary 1300 turns. Future supplies will probably include an anti-side tone induction coil.  The Magneto Bell and Cradle Switch are similar to the, C.B. type. The metal base plate at the rear is shaped to form a shelf which carries on its under side the moulded instrument Cord connection block and on a metal projection fitted to its upper side the magneto bell assembly and the cradle switch. The bell gongs are mounted on the upper side of the base plate directly in front of the metal shelf.

Hand Generator
The metal shelf projection also carries the small hand generator which is fitted with an “Alnico” permanent magnet. The generator is mounted with its driving axle at an angle of about 50 deg. to the metal base plate. When assembled in the telephone the generator driving axle to which the handle is fitted protrudes through the hole in the moulded dummy which is fitted in the dial aperture. The generator is screwed firmly to the metal projection and the metal base plate by five holding screws.

The recent introduction of a new magnetic alloy, aluminium-nickel-cobalt-steel, “Alnico,” to replace tungsten steel formerly used in the manufacture of permanent magnets has resulted in the development of a telephone hand generator of an entirely new design. “Alnico” has the maximum flux density per unit of volume for any magnetic material yet discovered. The specific gravity of the alloy is low also, giving a reduction in weight which is distinctly advantageous.

The very high starting force necessary due to the high magnetic flux and small air gap between the armature and the permanent magnet pole pieces has been a difficulty in connection with the development of a telephone hand generator using an “Alnico” permanent magnet. The force necessary when operating the generator on short lines of low resistance at the commencement of turning the handle was sufficient to move the table telephone in which the generator was fitted unless it was held firmly in position. The high efficiency of the generator has been retained and the turning force reduced by backing off the armature pole pieces at the trailing tip in order to reduce the flux density in the air gap and mounting the armature between pivots and collecting the current by means of small carbon brushes bearing on slip rings fitted one at each end of the armature.

The generator is provided with a malleable cast iron two pole armature running on pivot bearings and wound with enamelled wire to 500 ohms resistance. The brass driving wheel has 78 teeth and the steel pinion wheel 18, giving a ratio of 1 to 4.33. The pinion wheel may be withdrawn by removing the locking nut securing the pivot bearing which is accessible by the removal of the armature cheek. All moving parts of the generator other than the cut out spring set and the handle are totally enclosed. The cast “Alnico” permanent magnet is used for the bridge piece only, high quality soft iron pole pieces being provided. The generator cut out springs are fitted with double silver contacts. The weight of the generator is only 2 lbs. and its open circuit voltage at normal turning speeds is about 112 Volts. Fig. 8 shows the generator. disassembled. Fig. 9 shows the load characteristic curve of the generator compared to the present standard Hand Generator No. 4C (formerly No. 3).

Handset
The handset (see Fig. 10) is exactly similar to that provided on the C.B. type telephone with the exception that Ericssons Transmitter Inset N.7750 is fitted instead of Transmitter Inset No. 13. Ericsson Inset N.7750 is interchangeable with No. 10 and No. 13 types, and is an efficient type for local battery use, being suitable also for common battery operation. It is similar to No. 13 type in that the oiled silk membrane provided on the No. 10 inset is omitted, the diaphragm being. protected by a special lacquer. The inset has a :resistance of approximately 50 ohms and is provided with a completely enclosed carbon granule chamber to prevent the ingress of moisture.

C.B. Handset Table Telephone with Hand Generator (No. 336 C.B.T.)
A 300 type C.B. Handset Table Telephone with a hand generator fitted in the dial aperture in the body of the instrument will also be supplied. This instrument is for use as a C.B. extension telephone when an extension switch is at the main. In outward appearance the telephone is similar to the magneto type.

C.B. and Automatic Telephones - Circuit Description
C.B. and Automatic Handset Table Telephones No. 332 A.T. and 332 C.B.T. will be wired to Drawing C.1350 (see Fig. 11). The drawing shows the cord connections between the terminal strip and the telephone and between the telephone and the handset. When an extension bell is required, it is connected to terminals 1 and 2 of the terminal strip No. 20/4 and. the strap removed. When the telephone is required for C.B. working, the dial and dial cord are omitted and terminals 4 and 5 of the dial cord terminal strip are strapped together. Thee 0.1 microfarad condenser is connected across the transmitter to reduce radio pick-up interference to a minimum. The radio interference suppression unit for dial impulses is connected, if required, to terminals 2 and 5 of the dial cord terminal strip and the orange wire of the dial cord removed from terminal 5 and connected to the screw terminal of the unit designated O.R.

The following is a brief description of the operation of the telephone (see Figs. 11, 12 and 13). Fig. 12 indicates the dialling circuit.

(i.) Incoming Calls:
The ringing circuit is from line 1 through the magneto bell and 2 microfarad condenser to line 2.

(ii.) Originating Calls:
(a) Impulsing Circuit (Figs. 11 and 12) - The impulsing circuit is from line 1 through the cradle switch, dial off normal springs, dial impulse springs to line 2. A shunt circuit is provided across the dial impulse springs to prevent high voltage surges and correct impulse distortion, from line 1 through the magneto bell and 50 ohm non-inductive winding 5-6 of the induction coil in parallel through the 2 microfarad condenser back to line 2.

(b) Transmitter Feed Current Circuit - The transmitter is supplied with current from the exchange battery feeding bridge. The feed current circuit :is from line 1 through the cradle switch, induction coil winding 1-2, transmitter, dial impulse springs to line 2. The transmitter may be regarded as an alternating current generator.

(c) Sending Circuit (Figs. 11 and 13) - Considering the transmitter as an alternating current generator, the transmitter current divides, the major portion passing from the transmitter through the induction coil windings 7-3, 3-5, 5-6, the cradle switch springs, 2 microfarad condenser, dial impulse springs, back to the transmitter, the remainder flowing through winding 2-1 to line 1 and back through line 2 and the dial impulse springs to the transmitter.
As the current in winding 3-5 produces a greater magnetic flux in the core than winding 1-2, winding 3-5 acts as the primary of a step-up auto transformer in which windings 1-2 and 3-5 together form the secondary. The transmitter is, therefore, approximately matched to thee impedance of the line with a resulting increase in line cure-eat compared to a telephone without an induction coil. This increase in current is sometimes referred to as the booster effect.

(d) Sidetone Control - The flux produced in the core of the induction coil by current from the transmitter is the resultant of the opposing effects of windings 2-1 and 3-5. This flux induces an E.M.F. in the winding 3-4 and would produce sidetone current in thee receiver. One method of reducing this sidetone current would be to increase the ampere turns of 2-1 until they were approximately equal to those of 3-5. This, however, would destroy the auto, transformer or booster effect. To retain this effect, and at the same time reduce the sidetone the pon4nductive winding 7-3 is included in the local circuit of the receiver, and for ordinary line impedances, the potential across this winding opposes the E.M.F. induced in winding 3-4. Since the resultant flux from windings 2-1 and 3-5 depends upon the current in 2-1 which is controlled by thee impedance of the line, the amount of sidetone is influenced by the line conditions. The circuit is designed to give minimum sidetone under average line impedances. Fig. 6 shows the connections of the ASTIC No. 22.

(e) Receiving Circuit - In the receiving condition the major portion of the voice frequency passes from line 1 through the cradle switch springs, induction coil winding 1-2, transmitter, dial impulse springs, back to line
2. Due to the low impedance path offered by the transmitter compared to its parallel circuit formed by the induction coil windings 7-3, 3-5 and 5-6, cradle switch springs and two microfarad condenser, the flux due to the current through the induction coil winding 1-2 induces a potential in winding 3-4 which drives the Current through a local circuit consisting o the receiver and the induction coil winding 7-3. The small current from the line passing through winding 3-5 tends to assist the current passing through winding 1-2.

Magneto Telephone - Circuit Description
Magneto Handset Table Telephones No. 333 M.T. will be wired to Drawing C.1351 (see Fig. 14) which shows also the connections between the terminal strip No. 20/4 and the telephone, and between the telephone and the handset and the connections to the battery. If an extension bell is required, it is connected to terminals 1 and 2 of the terminal strip No. 20/4 and the strap removed.
Thee following is a brief description of the operation of the telephone:-

(a) Incoming Calls - The incoming ringing circuit is from line 1 through the magneto bell and thee break contacts of the generator to line.

(b) Originating Calls - When the generator is operated, the generator cut-out spring set disconnects the magneto bell and connects the generator across the line. The outgoing ringing circuit is from line 1 through the generator, and its make contact to line 2. Where thee handset is removed from the cradle switch the transmitter local circuit is completed from battery through the 1 ohm winding of the induction coil, transmitter and cradle switch back to battery. The receiver circuit is completed also from line 1 through the cradle switch, receiver, cradle switch, 19 ohm winding of induction coil, break contacts of generator to line 2.

C.B. Telephone with Generator - Circuit Description
The operation of the CB. Handset Telephone with hand generator, Drawing C.1352 (Fig. 15) will be readily understood from the descriptions of the C.B. and Magneto type telephones.

[EDITORS NOTE: In the December, 1937, issue of the “Journal” thee 232 type Handset Telephone was described. An article in a later issue will describe the 300 Type Telephone fitted with interlocking press button keys, trembling bell, etc., for use on various Plan Number Services.]

 

 
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