BELL No. 40A and 40B

Indoor D.C. Trembler bell, with Church style gong, used on fire alarm systems. 

The early Bells are wooden cased whilst the later bells from 1955 are plastic cased.  The wooden cover is secured by means of two flat brass side hooks.

Bell No. 40A - Twin 50 ohm coils, 2-10 volts - used  on London Fire Brigade systems.
Bell No. 40B - Twin 250 ohm coils, 10-50 volts - used in PO buildings and specified for omnibus call-bell systems.

Fitted with Bell-gong No. 21 (4.5in diameter x 3.1in - Church style).

The terminal strapping changes depending on the model of the Bell and the voltage to used - check the wiring diagram for the correct connections.

Winding Specification

  1. The coils shall be wound with wire, copper, enamelled to B.S.S. 156.  The conductor diameter, turns and resistance for the coils are shown in the Table below.
  2. The inner and outer coil ends of the Bell No. 40B shall be led out with 4/0.006 stranded copper wire, double silk covered. The colours of the covering shall be red for the inner end, and green for the outer end.
  3. Lead-out wires are not necessary for the inner and outer coil ends of the Bell No. 40A.
  4. The coils shall be wound anti-clockwise -when looking at the yoke end.
  5. The windings shall be protected with two layers of cloth, yellow varnished to Specification M1011 - secured by an approved cement.
  6. The resistance of each coil shall be shown on a paper label beneath.
Bell No. Conductor diameter Turns (each coil) Resistance (each coil)
40A 0.0108 ins. 3,460 2.5 ohms
40B 0.0068 ins. 6,000 12.5 ohms

Found in the Rate Book 1947 and 1956.

GPO Engineering Instructions on Fire Systems

Wiring Diagram - N640.

Drawing  - 90487.

Specification - S524.

Picture of wooden Bell taken 1937.  Plastic cased bells date from 1955.

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Last revised: March 16, 2024