Quartz Crystal

Natural Quartz crystals were used for oscillators in electronic equipment since 1921, and were the common means to determine transmitters’ frequencies from the 1930’s until late 1950’s.
Quartz crystal in a certain structure has a piezoelectric property – an electrical voltage applied to the crystal creates a mechanical force – or alternatively, a mechanical force creates an electrical voltage. The crystal behaves as a precise resonant circuit, at a frequency determined by the physical dimensions of the crystal.
Quartz crystals in military communication equipment were based on a natural crystal plate, cut at an suitable angle and mounted between two metal plates, which held it at its four corners, so that it could vibrate.