CLOCK No. 12

Clock No. 12

The Clock No. 12 is intended primarily for use in telephone exchange buildings.  It drives both the half minute and seconds indicating types of clock.  It is also capable of synchronising with Greenwich Mean Time.

This clock was regarded as the GPO standard master clock, but was superseded by the Clock No. 36.

Details of Mechanism
This Master clock has a seconds-beating pendulum and is used in exchanges in connection with both half-minute and seconds indicating clocks. The former are effected by means of a count-wheel A having 30 teeth on its periphery. This wheel is propelled by means of a paw B attached to the pendulum rod C, which rotates the  count wheel A one tooth for each complete swing to and fro of the pendulum.

On the count-wheel A are two indentations diametrically opposite each other, which permit the pawl B to drop lower and engage the push rod D at every half minute.  This push rod is attached to a contact spring E which, on being closed, permits an electric impulse to be sent out at every half minute.

At the top of the pendulum rod C and near to its point of suspension a fitment is attached which alternately closes a contact on either side, sending out one electric impulse every second. These two contacts FF are connected in parallel.

The clock is fitted with the synchronizer G which operates as follows:-
The passage of the Greenwich Mean Time (GMT) current through the electromagnet of the synchronizer causes its armature H to be attracted to the left armature at its upper end engages a small roller attached to a bell-crank lever J, the long arm of which is caused to travel upwards to meet the heart-shaped cam K on the count-wheel A.  This cam, when pressure is applied, to it by the bell-crank lever, causes the count-wheel to return to zero from either direction, according to whether the wheel is within 14 seconds fast or slow of Greenwich Mean Time.  At the same time, the movement of the inclined plane L which is attached to the roar portion of the bell-crank lever causes a smaller bell-crank lever situated behind the count-wheel to lift the propelling pawl B and also a retaining pawl, thus disengaging time pendulum from the count-wheel.  The heel of the smaller bell-crank lever is shown in the illustration.

On the release of the armature of time synchronizer by the cessation of the time current the various clock movements are resumed at correct Greenwich Mean Time.

The pendulum is maintained in vibration by means of an electromagnet M, having extended pole pieces N, embracing the pendulum rod.  Situated above these and attached to the pendulum rod is an iron armature P.  The electromagnet M is energized by currents from a 4-volt battery controlled by the contacts Q.

So long as the are of oscillation of the pendulum lies outside a pre-determined minimum, the detent R trails over the notched agate S attached to the pendulum rod.  When the arc has declined to the minimum the detent R engages in the notch, and on the return swing of the pendulum, acts as a toggle, raising the contact spring X so that the driving battery is momentarily joined to M, thus giving an impulse to the pendulum.

The clock must be vertical, and to ensure this a register T is provided at the lower extremity of the clock case, such that when the clock is in the correct position the point V is directly over T.

A rating nut W is provided, by means of which the clock call be adjusted to have. a slight gaining rate not exceeding 14 seconds per day.

The arrangement of contact springs adopted is such that the external circuit closed through a resistance before the battery spring breaks its contact. This lever prevents sparking at the main contact.

Superseded the Magneta clock system.
Supersedes the  Clock No. 16.
Superseded by the Clock No. 36.

Diagrams - GMT24 & GMT25.
Diagram when used for driving Clocks, Electric No. 28 - GMT8.



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Last revised: January 07, 2024