PAGE No. 34

A New Magneto Table Telephone
January 1939

During 1935 the Ericsson Company introduced entirely new designs of moulded case type table and wall telephones suitable for auto/C.B. working. These instruments had the outward appearance shown in Fig. 1 & 2.

Fig. 1
Auto. Table Telephone
Fig. 2
C.B. Wall Telephone

The popularity of this type of one piece moulded case was immediately apparent and further developments were subsequently undertaken with a view to increasing the scope of this type of telephone. Certain improvements in transmission and reception technique had become available during this period, and these improvements were incorporated in the new range of instruments.

The outcome of these further developments, which were undertaken in collaboration with the engineers of the British Post Office, was a type of telephone which can be used for many purposes by interchanging a few pieces of standard apparatus. This new type is now adopted and standardized by the British Post Office.

Fig. 3
Auto. Telephone with Key Units
Fig. 4
C.B. Telephone Showing Sliding Tray

The main features of the new design auto/C.B. table telephone are improved transmission efficiency of approximately 1 d.b. above the British Post Office standard fixed for telephone No. 162 and bell set No. 25, whilst reception is equal, and the inclusion of a new anti-side-tone induction coil providing much better side-tone conditions. Besides the foregoing improvements several key units were devised which can be added to the standard instrument when extension and operator-recall facilities are required. A further convenience was the provision in the telephone base of a sliding tray carrying a writing pad intended as a ready reference for the subscriber’s personal use. Fig. 3 & 4 show views of this telephone and its outstanding improvements.

This latest design is now standardized by the Ericsson Company and all the usual telephone requirements can be met by this style of telephone, even to the inclusion of magneto working, using only the comparatively small one piece instrument in conjunction with the usual external battery.

Hitherto telephones for magneto working have been heavy and cumbersome on account of the ungainly nature of the magneto generators available. Recent research has produced new magnetic materials which the Ericsson Company have incorporated in a new and highly efficient magneto generator of very small size and weight. This new generator is shown to the right. It will be seen that the characteristic load curve for the new generator shows an output approximately twice that of the British Post Office No. 4C type and with this gain is associated a 25% decrease in size and weight. A slightly larger generator is available giving an output equal to the heavier B.P.O. No. 8A type.

Fig. 6
The Generator Size compared with two others 

This small generator was found to be eminently suitable for inclusion in the newest type moulded telephone case as used on the auto/C.B. telephones mentioned above, consequently further developments have resulted in a type of table telephone suitable for magneto working which is small in size and weight and at the same time pleasing in appearance.

Fig. 8
The New Magneto Telephone

The new magneto telephone is illustrated in Fig. 8 (Telephone N2120). The standard auto/C.B. moulded case is used, and in addition to being a cover for the components, provides a cradle for the moulded micro-telephone. A metal base plate carries all the internal apparatus and is completely detachable from the case-work. This base with components is illustrated in Fig. 9. On it are a condenser, induction coil, generator, ringer, ringer gongs, cradle switch springset and cord connection block. The condenser is, of course, only required in particular instances and is not fitted in standard magneto telephones. The illustration, however, shows that space has been provided for its inclusion.

In view of the space required by the generator and other components, the sliding tray fitment provided on auto/C.B. types is not included, the aperture in the casework is therefore closed by means of a moulded strip fixed to the front edge of the base plate.

The dial aperture in the casework is used to accommodate the generator spindle and handle. A moulded escutcheon is provided which fixes in the aperture in the same manner as a dial or a dial dummy, and is arranged so that its centre hole can be brought concentric with the generator spindle. The escutcheon also carries a label holder of the type provided on dials and dial dummies.

The generator handle is fixed by a single screw and this must be removed before unscrewing the six captive screws in the base plate when it is desired to obtain access to the interior of the telephone.

Fig. 9
Base Plate Removed showing Internal Apparatus
Fig. 10
Underside of Base Plate

The underside of the base plate is shown in Fig. 10 and illustrates the means provided for giving easy access to the generator cut-out and to the cord terminal block. It will be seen that subsidiary cover plates provided with captive fixing are employed, so that it is only necessary to remove these for immediate access to the cord connections and generator cut-out.

A noteworthy feature of this new set is the micro-telephone, which is identical with that used on the auto/C.B. instruments. In fact this handset is common to all these modern moulded case type telephones, any difference in working conditions between CB and LB being covered by the induction coil, whose physical dimensions are the same in each instance, the variation in requirements being covered by the windings only.

The approximate weight of the new magneto table telephone is 7.5 lb., which is 4 lb. lighter than instruments of similar efficiency of the older types.

Telephones destined for use in tropical climates are given special attention, and precautions are taken to provide good insulation for these exacting conditions. Also the ingress of destructive insects through the ringer sound holes etc. is prevented by means of metal gauze covers.

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Last revised: October 14, 2019