Hofmann voltameter. The Hoffman voltameter is an apparatus designed by German chemist August von Hoffman in 1866 to demonstrate electrolysis of water. Three glass tubes are connected by a bridge near their bases. The middle tube is used to fill the apparatus with water and a small amount of electrolyte (e.g. dilute sulphuric acid). Each of the other tubes has a platinum electrode sealed into its base and is stoppered at the top. When a current is applied to the electrodes, hydrogen is liberated at the cathode and oxygen at the anode in the volume ratio of 2:1. If both gases are diatomic (which they are), this shows that water contains twice as many hydrogen atoms as oxygen - H2O.