millicoulomb

The millicoulomb sits between the static-electricity scale and the battery scale, and it is easiest to grip through the solvers that produce it. One milliampere flowing for one second is one millicoulomb by $Q = It$; a photoflash capacitor of 1000 µF charged to 300 V holds 300 mC by $Q = CV$; and a single milliampere-hour, the smallest increment a battery datasheet bothers with, is already 3600 mC. Coulometry, the analytical chemistry technique that measures a substance by the charge needed to convert it, reports at this scale as well.

The unit also carries a fine piece of measurement history: charge used to be weighed. From 1908 the international ampere was defined by electrolysis, as the current that deposits 1.118 milligrams of silver per second from silver nitrate solution, which makes one millicoulomb exactly 1.118 micrograms of silver on the cathode. National laboratories realised the electrical units with silver voltameters, literally weighing metal to determine charge, until the definition was retired in 1948 in favour of the absolute ampere. The site's Faraday electrolysis solver runs the same physics in the modern direction, from charge to deposited mass, with the exact 2019 constants underneath.