Mains frequency (North America)

Sixty hertz is an engineering compromise that hardened into a continental standard. Westinghouse's engineers, working with Nikola Tesla's polyphase patents in the early 1890s, wanted a frequency high enough that arc and incandescent lamps would not flicker visibly and transformers could stay small, but low enough to limit reactive losses in long lines. Charles Steinmetz's team at General Electric settled on 60 Hz, and Niagara Falls generation locked it in for the continent.

The frequency is a live measure of the grid's power balance: if load exceeds generation the spinning turbines slow and frequency droops, so operators watch it second by second and let it wander no more than about 0.05 Hz in normal operation. It also propagates into everything downstream — synchronous motor speeds of 3600, 1800 and 1200 rpm, transformer core sizing, mains-hum notch filters at 60 Hz and its harmonics, and the once-common practice of synchronous clocks counting cycles.