A Hofmann voltameter is an apparatus for electrolysing water, invented by August Wilhelm von Hofmann (1818–1892) in 1866. It consists of three joined upright cylinders, usually glass. 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. When current is run through Hofmann's voltameter, gaseous oxygen forms at the anode and gaseous hydrogen at the cathode. Each gas displaces water and collects at the top of the two outer tubes.
The tubes which collect the product gases are usually graduated, so that the volume of gas can be measured.
