<P> The name' voltameter' was coined by Daniell, who shortened Faraday's original name of "volta - electrometer". Hofmann voltameters are no longer used as electrical measuring devices . However, before the invention of the ammeter, voltameters were often used to measure direct current, since current through a voltameter with iron or copper electrodes electroplates the cathode with an amount of metal from the anode directly proportional to the total coulombs of charge transferred (Faraday's law of electrolysis). The modern name is "electrochemical coulometer". Although the correct spelling of Hofmann contains only one' f', it is often incorrectly depicted as Hoffmann . </P> <P> The amount of electricity that has passed through the system can then be determined by weighing the cathode . Thomas Edison used voltameters as electricity meters . (A Hofmann voltameter cannot be used to weigh electric current in this fashion, as the platinum electrodes are too inert for plating .) </P> <P> A Hofmann voltameter is often used as a demonstration of stoichiometric principles, as the two - to - one ratio of the volumes of hydrogen and oxygen gas produced by the apparatus illustrates the chemical formula of water, H O. However, this is only true if oxygen and hydrogen gases are assumed to be diatomic . If hydrogen gas were monatomic and oxygen diatomic, the gas volume ratio would be 4: 1 . The volumetric composition of water is the ratio by volume of hydrogen to oxygen present . This value is 2: 1 experimentally; this value is determined using Hofmann's water voltameter . </P>

Apparatus used to determine the volumetric composition of water