Hofmann voltameter

The inner cylinder is open at the top to allow addition of water which contains a low concentration of a compound such as sulfuric acid to improve conductivity and complete the circuit.

A platinum electrode is placed inside the bottom of each of the two side cylinders, connected to the positive and negative terminals of a source of electricity.

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).

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, H2O.

The volumetric composition of water is the ratio by volume of hydrogen to oxygen present.