Hertz (Hz) - Electrical
Modern equivalent
Cycles per second
Named after Heinrich Hertz, tracking alternating current wave cycles.
Measuring the frequency of periodic waveforms, the hertz became the standard metric for electrical grid frequencies and radio wave transmissions. Britain's National Grid holds its alternating current at precisely 50 Hz, and does so with such consistency that generations of household electric clocks kept time by counting the pulses of the mains supply itself rather than by any internal mechanism.