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The Synchronous electric clock. Now, not very often used as a main timekeeper in a household having been replaced by Quartz based movements in recent decades. During the 30s, 40s, 50s and into the 60s electric synchronous clocks were ubiquitous. Their accuracy is maintained by the mains frequency which is kept extremely accurate by the national power generating companies. Over the long term these clocks are the most accurate.
Look at the 'Real Time Frequency Data'. The frequency of the AC mains controls the speed at which an electric motor runs and so maintains timekeeping via wheelwork to drive seconds, minutes and hour hands.
Synchronous Electric Clocks & Impulse Driven Slave Clocks
All synchronous clocks work on the mains voltage/frequency of the country in which they were designed for. Many of these clocks have features which would not pass modern electrical safety standards and in some the insulation of wires carrying mains voltage breaks down.
For this reason some of the clocks that I sell I modify to work at 9vac rather than mains. This involves a simple rewind of the coil driving the motor and a 9v power supply unit. All other aspects of the clock are left original.
The frequency of the mains controls the speed at which the clock runs. For this reason a clock made for the UK or Australia will not run at the correct speed in the US and vice versa.
The US mains is 60Hz and UK and Australia mains runs at 50Hz - rather annoying for the clock collector.
electric-clocks.co.uk has developed a frequency converter that will convert 50Hz to 60Hz and run USA clocks in the UK and Australia and other coutntires on 50Hz mains.
There is also a version to run an English clock in the USA.
A small circuit controlled by a crystal and micro-controller and giving either a 1 second, 30 second or 60 second pulse to drive a slave clock in the absence of a Master Clock. Small enough to fit behind the clock.
The controller activates a relay at the desired interval. The clock can then be powered by a battery pack located in the back of the clock or by an external wired power supply if desired. It will come as a bare built circuit, mounted or in a plastic enclosure.