Sources of Electromotive Force

Author: E.E. Kimberly

When a sheet of copper and a sheet of zinc are immersed in a dilute solution of sulfuric acid, an electromotive force is produced and a difference in electrical potential is found to exist between the metal sheets. When these two sheets of metal are connected by a conducting wire, electrons flow through the wire from the zinc to the copper while within the solution electrons flow from the copper to the zinc. Thus, the electrons are said to flow around the circuit. Many combinations of other metals and other solutions will produce the same phenomenon involving flow of electrons. Electromotive force is thus produced by chemical action. This is the principle of all primary batteries, including the so-called dry cells used in pocket flashlights.

When a rod of a metal such as bismuth is joined to another rod of a metal such as antimony and their joined ends are brought to a temperature different from that of their free ends, an electromotive force is produced and a difference in electrical potential appears between their free ends. Such a combination is called a thermo-couple. Similar use of many other metals produces similar results. Thermo-couples find use in the measurement of temperatures, particularly those for which mercury-in-glass thermometers would be too sluggish in response or where it would be inconvenient or impossible to place them satisfactorily.

The most common method of producing or generating an electromotive force is that of moving a conductor across a magnetic field or by causing the strength of a magnetic field to change in a coil of a conductor. These phenomena will be explained independently.