This Master clock was made by English Clock Systems Ltd around 1949. English Clock Systems Ltd was part of Smiths English Clocks Ltd based in North London. The clock has a glazed wooden case and can drive a large number of half minute slave dials connected in a series loop, the dial in the case being the first of those. Impulse to the pendulum is given by a weighted arm falling onto a curved sloping impulse face attached to the pendulum. A battery is required to reset the gravity arm on to its latch, and to impulse the slave dials.
The clock is quite noisy in operation, but is just acceptable in a home
environment.
The pendulum is suspended from the top of a cast iron 'A' frame which also supports the gravity arm and resetting coils.
The one second pendulum is made of Invar with a substantial cast iron
bob.
The two coils that reset the gravity arm can be seen on the right of the pendulum. As the gravity arm falls, an electrical contact is made that energizes the coils to reset the arm. Also in series with the resetting coils are all the slave dials, so the slaves only receive a pulse to advance them when the gravity arm is reset.
At the top of the case is a terminal block for the internal wiring of the clock, and at the top left is a metal-cased 2uF capacitor to help quench the spark across the contacts. At the top right is a variable resistor which is in series with the resetting coils and slave dials, and is used to set the current drawn by the whole system to 320mA, irrespective of the battery voltage or number of slaves.
The gravity arm is released every 30 seconds. A 15 tooth escape wheel is indexed round by one tooth each time the pendulum swings left to right, i.e. once every two seconds. Behind the escape wheel and fixed onto its arbor is a wire (visible diagonally from 1 o'clock to 7 o'clock), which releases the catch holding the gravity arm once each revolution of the wheel, and triggers the whole resetting and impulse action. The weighted gravity arm is caused to fall; the roller rides down the curved plane attached to the pendulum, and impulse is given to the pendulum; at the bottom of its fall, the electrical contacts (on the right of the coils) are made, the arm is reset, and the slaves are pulsed.
This is the slave movement in the master clock. The electromagnet attracts its armature (on the right), which is pivoted at the lower end of the diagonal bar, and releases it after about 1/5 second (the duration of gravity arm resetting). As the armature is released, the indexing pawl at the top pushes the large 120 tooth wheel round by one tooth and thereby moves the minute hand by half a minute. The hour hand is driven by conventional motion work. The silver coloured lever across the top stops the minute hand being driven backwards.