Apothecaries’ Ounce (℥)

The dispensing ounce of the pharmacopoeia, identical in weight to the troy ounce but subdivided entirely differently.

The apothecaries’ ounce weighs exactly what a troy ounce weighs, and the two are the same quantity reached by different arithmetic: the goldsmith counted twenty pennyweights to his ounce, the apothecary eight drachms, or twenty-four scruples, or 480 grains. The difference is not pedantry but purpose, since a prescription had to express ratios that a bullion weight could not. Its symbol, a stylised script character with a stroke through it, was written on every prescription in Britain until apothecaries’ weights were abolished for dispensing in 1971, and it survives now mainly in historical pharmacy records and in fiction that wants to sound faintly sinister.