Standard Cell

—A special form of primary cell used to establish the normal value of the volt. The Clark cell was for a long time universally used as the standard with an E. M. F. of 1.434 volt at 15 degrees C. An improvement known as the Carhart- Clark cell had an E. M. F. of 1.44 volt. The cell now recognized as the standard is the Weston cadmium cell containing electrodes of cadmium amalgam covered with a layer of cadmium sulphate crystals, and pure mercury in contact with a paste of mercurous sulphate, cadmium sulphate crystals and metallic mercury; the electrolyte being concentrated solution of cadmium sulphate and mercurous sulphate. The Weston cell has an E. M. F. of 1.0186 volt between 5° and 26° C. Its chief advantages over the Clark cell are its much lower temperature coefficient and longer life.