PAGE No. 36

Bakelite Telephones
January 1934

RECENT years have witnessed a notable change in the design and construction of telephone instruments, due largely to the application of mouldings of the synthetic resin type to their manufacture.

By the application of modern moulding methods, an instrument of the type shown above (N1010 shown) was developed early in 1930. This instrument is of the one-piece, self contained type, i.e. the components which sometimes form a separate unit - a bell set - are incorporated in the instrument.

For many years an instrument with separate transmitter and receiver was largely in vogue with telephone authorities, but with the introduction of moulded Bakelite telephones as illustrated, a general return is taking place to an earlier practice in favour of the hand micro-telephone, a unit originally designed by the Ericsson Company.

The number of parts is kept down to a minimum, and in consequence the quantity of spares which it is necessary to carry for repair and maintenance is reduced, thus effecting an economy in maintenance costs.

Easy access to the internal apparatus is effected by simply unscrewing the four screws fixing the base plate; the screws are of the captive type and are retained in the plate. On the base plate are mounted the ringer, condenser and connecting terminal block (the equivalent of the bell set which it is customary to use with the pedestal type of telephone). Connections are made to the cradle switch springs, anti-side tone induction coil, and automatic dial fitted in the body of the instrument, by means of a three way flexible cord of sufficient length to enable the body portion to be lifted clear of the base, thus giving ready access to all the components.

The moulded body, on which the micro-telephone cradle and plunger operating the switch springs are fitted, has an aperture for accommodating an automatic dial of the standard British Post Office type. This aperture is fitted with a suitable cover plate when the instrument is required for manual C.B. operation.

The instrument is also readily adapted for wall fixing by the use of a specially designed bracket. When used in this manner it is customary to dispense with the three-way flexible cord and moulded connecting block, normally used when the telephone is for desk use, the external wiring being connected direct to the instrument terminals.

The hand micro-telephone is of the Bakelite moulded type and is fitted with a three-way flexible cord of suitable length. The connectors between the receiver and transmitter housings are embedded in the moulding. The receiver, utilizing a cobalt-chrome steel magnet, is of the inset type, and its electrical connections are automatically completed by means of the fixing screws. The capsule type transmitter has been specially designed and incorporates highly polished immersed electrodes. It is readily repairable, the diaphragms being clamped in position by means of screwed rings. The connections to the transmitter are also automatically completed, when the inset is placed in position, by means of a spring pin in the transmitter housing of the moulded handle engaging with a socket connected to the bottom electrode of the inset, and a fixed flat type spring making contact on the case.

In the event of the instrument being required for use under adverse climatic conditions, special finishes are given to all metal components in order to prevent corrosion, all coil windings are suitably impregnated, and cords specifically designed for use under tropical conditions are fitted.

Increased transmission efficiency is ensured by the use of a specially designed induction coil embodying the anti-side tone feature, shown in the circuit diagram, (A reproduction of this diagram is affixed to the top of the condenser case in each instrument, for purposes of reference, making connections and tracing faults). The combination of the anti-sidetone circuit and the improved capsule transmitter provides a high standard of transmission. The average efficiency of transmission is 5 decibels better than that of the C.B. solid back transmitter as specified by the British Post Office and the average efficiency of reception is 0.5 decibel below that of the bell receiver standard, also specified by the British Post Office.

The general design of the instrument follows closely that of the latest British Post Office standard, and as far as practicable, many of the components are interchangeable with those of the Post Office Instrument (Type No. 162).

One of the interesting features is a ringer having the usual standard size gongs inclined for mounting in a restricted space, as may be seen from picture above. It utilizes many components of the British Post Office standard Bell No. 1A and is so designed that with minor modifications, this bell can be readily converted for use in the instrument. This adaptability should make a special appeal to telephone administrations, who have stocks of these bells, and who desire to provide their subscribers with the ideal one-piece instrument and its accompanying increased efficiency and ultimate economy. For this purpose the Ericsson Bakelite telephone can be supplied, with the base plate unequipped, so that it may readily replace the pedestal type and be temporarily connected to the existing separate bell set, the anti-side-tone features of the combination induction coil being at the same time utilized.

It will therefore be seen that a change over to the self-contained instrument may be made gradually by slight modifications to the method of mounting the gongs of the bell set ringer, thereby enabling the latter to be fitted on the originally unequipped base, the condenser from the bell set, when of suitable size and capacity, may also be readily fitted to the base.

From the foregoing brief description of the Ericsson Bakelite instrument, N1010A type, it will be seen that the telephone is suitable for service under the most exacting conditions, its high transmission qualities and low cost of maintenance rendering it suitable for use by telephone administrations in all territories.

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Last revised: February 08, 2022