Convert hertz to millihertz

The hertz replaced 'cycles per second' in 1960, and old American texts still write cps or c/s. Mains electricity is 50 Hz in Europe, Asia, Africa and Australia and 60 Hz in the Americas (Japan has both). Human hearing spans about 20 Hz to 20 kHz, concert pitch A is 440 Hz, medium-wave radio sits at hundreds of kilohertz, FM between 87.5 and 108 MHz, Wi-Fi at 2.4 and 5 GHz, and processors tick at 3–5 GHz. The fresnel, 1 THz, was proposed for optical spectroscopy and never caught on; visible light itself is 400–750 THz.