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Waterproof Telephone Set
for Electric Trams and Poles Supporting
High-Voltage Conductors
(Magneto Calling and Local Battery)
No. 1278A
This device was designed and assembled
for use on telephone lines with three points attached to poles supporting
conductors carrying a medium voltage of 500 volts. The proximity of these
conductors is very dangerous for telephone lines; therefore, it is necessary
to provide an effective protection system against accidental contact.
The trolley poles of a tramway network
must be classified in this category since the electromotive force they carry
is rarely less than 500 volts.
The telephone set is enclosed in a
cast iron case.
The protection system consists of two fuses, two
carbon switches, and a transformer coil designed to completely isolate the
telephone set from the external circuit. This coil comprises two windings
made of wires covered with insulation of several megohms in resistance.
The outer sheath of the connecting cord is grounded.
The resistance of the fuses is
calculated for a specific current intensity, resulting in immediate blowing
of the fuse and automatic disconnection of the device when contact occurs
between the telephone line and a high-potential conductor.
The carbon switches also operate in
short circuit (Mullein) and bring the external circuit into direct contact
with ground.
Replacing the fuses presents no
difficulty.
The telephone and local battery are switched off by means
of two push-button switches, the operation of which is controlled by the
door of the cast-iron box.
As the unit can be easily removed from the
box, inspections or repairs are very convenient.
The cast-iron box is
locked with a key, but the lock mechanism has a peculiarity:
the key
cannot be removed while the door is open.
Two cylindrical dry cell
batteries (Blue Bell) can be placed inside the unit, but these are only
supplied upon the customer's express request.
1278
A Telephone
Set, cased in Cast Iron, comprising a magnetic generator with 5 magnets No.
47G, transducer coil No. 25E, induction coil No. 13, Solid Back
transmitter No. 244W with receiver No. 131W and handle, cord No. 242
of 46cm length, cord No. 243 of 20cm length, 2 x 1amp fuses, 2 carbon
switches and 2 automatic switches operated by the door of the cast iron box.
Dimensions: 410 x 300 x 300mm.
Taken from the 1910 BTMC Catalogue
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BELL
TELEPHONE MANUFACTURING Co.