Clock backup from the early days of mains power, c.1934

Overview

DAC Chronometer in polished teak case with glazed front panel and 7-inch pilot dial, advance-retard mechanism and auto-maintaining power. Manufactured by Gent, this clock arose from a very specific set of circumstances: widespread AC mains supply had prompted the use of clocks powered from synchronous motors; at the same time, it was not yet possible to rely entirely on a system still subject to periodic outages.

This clock combines two separate field and armature systems: one for AC and one for DC – hence its name, ‘DAC’. While AC supply is available, the DC maintaining device is inoperative, but if the mains power fails, the clock could switch the whole circuit to DC operation, thus keeping the subsidiary clocks running for the duration of the outage. The advance and retard switch additionally allows the clocks in the circuit to be adjusted for British Summer Time: setting the advance switch causes the chronometer to impulse all the dials every 2 seconds instead of at half-minute intervals, thus adjusting one hour in four minutes.

Since increased mains reliability quickly made this clock obsolete, this is a very rare survivor from a short period in horological history.