A rod of copper in copper sulphate solution comprises another half cell. In this case the spontaneous reaction is one in which copper ions in solution take electrons from the electrode and are deposited on the electrode as copper atoms. In this case, the copper acquires a positive charge. The resulting cell can then supply current if the electrodes are connected through an external circuit. This is the direction in which the cell reaction occurs for a positive e.m.f.
Concentration cells are cells in which the e.m.f. is caused by a difference of concentration.
This may be a difference in concentration of the electrolyte in the two half cells. Various types of voltaic cell exist, used as sources of current, standards of potential, and experimental set-ups for studying electrochemical reactions. See photoelectric cell . See solar cell . 1. half cell A Dictionary of Science, Oxford University Press 2.