This is a Synchronome Programmer made, not for use with their range of master clocks, but for the GPO 'Clock Nos.36' Hipp Toggle master clocks used in telephone exchanges. The basic function of the unit, correctly called a 'Clock 70A', is to receive a 30 second pulse and then arrange for this unit to send out a pulse every hour and also a single pulse once a day. These pulses being used to control timing and maintenance equipment.

The 30 second pulses that drove this programmer, and also the exchange slave clocks, was obtained from a separate relay set, a Clock Unit GMT 34: a typical convoluted name that the General Post Office seemed to thrive on.
It is that relay unit, the GMT 34, that the master clock actually sent its pulses to. The reason for this arrangement is that the contacts in the master clock are not capable of handling the current that several slave clocks and a programmer would demand. The relay it operates in the GMT 34 however only has a resistance of 5000 ohms, meaning the contact life is prolonged.

The mechanism consists of an electro-magnet and a pawl and ratchet arrangement much the same as the slave clocks's operation.

For a Close Up of the Cam