Voltameter used in the electrolysis of water, 20th century.

Voltameters are instruments used to measure the quantity of electric charge in a circuit, and were often used to demonstrate how gases are produced by electrolysis. Michael Faraday (1791-1867), Sir Humphry Davy's succesor at the Royal Institution, introduced the instrument in 1834, originally calling it the 'volta electrometer', because the amount of gas produced is proportional to the quantity of electricity produced by a voltaic pile. This term and its shortened form, voltameter, are now effectively obsolete; the modern term 'coulometer' was introduced by Theodore William Richards in 1902.