A cell that has a substantially constant terminal voltage and is used as a reference standard for electromotive force. This cell is constructed in an H-shaped glass vessel. The positive electrode is mercury and the negative electrode is a cadmium and mercury amalgam with a saturated cadmium sulphate solution as the electrolyte. The e.m.f. developed by the cell at 20 °C is 1.018  58 volts. The cell has a very low temperature coefficient of e.m.f. Compare Clark cell.