Candlepower (cp)

Defined by the Metropolitan Gas Act 1860 against a pure spermaceti candle of specified weight and burn rate.

Victorian Britain needed to measure light because it was buying it. Gas companies sold illumination, and the Metropolitan Gas Act of 1860 gave the buyers something to hold them to: one candlepower was the light produced by a pure spermaceti candle weighing one sixth of a pound and burning at one hundred and twenty grains an hour.

Every part of that is specific. Spermaceti is the waxy substance taken from the head of a sperm whale, chosen because it burns more evenly and cleanly than tallow. A sixth of a pound is 1,166 grains, so at the prescribed rate the standard candle burned for a little under ten hours. The burn rate is in the definition because a candle burning faster gives more light, so weight alone would not have pinned it down.

It is a thoroughly physical way to define a unit, and thoroughly of its moment: an Act about gas, regulated against whale oil, in the decade before electricity arrived. The modern candela is its direct descendant, and one candlepower is 0.981 of one.