Bank of Weston cells. These electrochemical cells produce a stable voltage used as a standard to calibrate voltmeters and define the SI unit for electromotive force (the volt). Also called a standard cell, it were invented in the USA by physicist Edward Weston in 1893. They have cadmium-mercury anodes (top of glass tube), with cathodes of liquid mercury (bottom of tubes) covered by mercury sulphate (white) and mercury. The electrolyte is a clear solution of cadmium sulphate. Weston cells provided the standard for voltage measurements between 1911 and 1990. Photographed at the National Physical Laboratory, Teddington, UK.