No. 23 |
PAGE No. 27
New Magneto Telephones
The main principles of design of magneto telephones developed by the company or use in coal mines have not materially changed in the last thirty years, but improvements have been made from time to time as a result of experience and advances in technical knowledge. In conformity with this progressive policy, two new telephones have been developed to replace the present standard sets N2974 and N2984.
The new instruments are coded N2976 and N2986, and, like their respective predecessors, are practically identical except that the former has a hand micro-telephone and the latter a fixed transmitter with separate receivers, consequently, other details of their construction given herein may be regarded as applying to both.
To give some idea of the changes which have been made in the appearance of the ironclad magneto mine telephone over the last thirty years, illustrations of equivalent instruments manufactured in 1921, 1934 and 1950 respectively, are shown in Figs. 1, 2 and 3. Nothing will be gained from a detailed comparison of the telephone in Fig. 1, with those in the other illustrations, but in view of the widespread use and popularity of the N2984 instrument, it will be compared with its superseding equivalent N2986.
Two of the primary requirements of telephones for use in coal mines are that they shall be capable of withstanding very rough usage and shall be well protected against damage from knocks, falling roof material, etc. This originally led designers to favour a rather too massive construction, with the result that a certain amount of difficulty was created for the installer and the maintenance electrician, owing to the considerable weight and size of the instruments. These points have been borne in mind in the designing of the new sets, as the following comparison reveals:-
Old Type, N2984
New Type, N2986
The reductions have been made without prejudicing the strength, safety or efficiency
of the instruments ; indeed, by using improved types of components and modifying the design, increased speech efficiency has been achieved.
The interior construction is revealed in Fig. 5 and it will be observed that the whole of the apparatus, with the exception of the batteries and terminals, is mounted on the inner door which is gate-hinged and thus constitutes a comparatively light, removable apparatus unit, when the wires from the
The transmitter housing is extended upwards and forms a strong support for the receiver arms pivotted on either side. These move in unison and carry 40-ohm inset receivers set at an angle from the vertical plane in order to provide a more comfortable fitting for the head, while the horizontal distance between them can be varied as necessary. The cast iron cradles formerly used, to protect the receivers from damage when the arms are released from the raised position, are replaced in the new instrument by resilient rubber blocks.
Below the transmitter is the generator crank which has a bearing designed for long service and a large aluminium handle.
At the rear of the inner door are two compartments; the lower one has a domed aluminium cover and contains a modern single-magnet “alnico“ generator with a non-inductive shunt across the armature winding, while the upper compartment, shown open in Fig. 6 (not shown), has a brass coverplate hinged at the bottom. Mounted on the outside of the plate are a heavy-duty ringer and a 5-way bakelite terminal strip which projects through to form the connecting point for the internal wiring. On the inner face of the plate are an anti-sidetone induction coil, a blocking capacitor, and a safety capacitor for the ringer, while inside the chamber are the transmitter, the receiver switch, and a 2-way bakelite terminal block for the receiver connections which are made in P.V.C. insulated flex threaded down the arms. The switch springs are operated by a cam when the receivers are raised.
The interior of the main case (Fig. 5) is horizontally bisected by a metal wall, the lower portion housing two BSS.397, type DR2, dry batteries, and the terminal blocks. As batteries are liable to cause
corrosion if allowed to become exhausted, their isolation is a desirable feature of the new design. They are supported on projections formed in the casting, so that they will not be affected, should condensation cause water to form in the bottom of the telephone. A similar precaution is taken with the line terminals which project from the terminating chamber through a thick bakelite strip in the battery compartment.
The line terminating chamber, similar to that on N2984, has three entries, one being fitted with a plug and two with glands for armoured cable. Plug and glands are interchangeable so that the external leads can enter the telephone from either side or from below. Glands suitable for
0.75“ screwed conduit can also be provided.
This necessitates some modification of the inner door construction, as may be observed from Fig. 7.
Last revised: October 14, 2019