The standard cell is used as a reference voltage when measuring potentials with a potentiometer.
The original design is due to the British engineer Latimer Clark, ca. 1878. The idea was to make a reproducible standard of EMF that could be used for practical work in electricity. The original design used zinc and mercury electrodes immersed in zinc and mercury sulphates. The English-American inventor and electrician, Edward Weston (1850-1936), patented an improved standard cell in 1893 that replaced zinc with cadmium. This cell's EMF changed very little with temperature, and it had a characteristic EMF of 1.0183 V at 20ºC.
This cell, in the Greenslade Collection, was made by the Eppley Laboratory of Newport, Rhode Island.