Quartz crystal clock, 1942.

Greenwich Observatory quartz clock equipment, 1942.
This clock was made at the Reseach Station of the British Post Office in Dollis Hil, London. It is similar to the clocks which replaced pendulum clocks as the time standard at the Royal Observatory in 1942. Its accuracy is of the order of a thousandth of a second per day and it utilises the vibrations of a quartz crystal, which is very high, 10,000 vibrations per second in this instance. The vibrations produce electrical effects which in this clock are divided electronically to produce a current which pulses 50 times per second. This is used to operate an electromagnet which drives a conventional clock dial. Because of the limited life of the electronic components available at that time it actually consists of three complete units whose performance can be compared with each other, enabling a defective unit to be detected and repaired.
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