The measurement of the intensity of an electromagnetic field is a matter of great practical difficulty. However, as the movement of a wire in any magnetic field tends to set up a current in the wire, and as the field may be that of a permanent magnet, or even that of the earth, and since, also, the strength of any field is proportional to the number of its lines of force per unit area, while the current generated in a wire is also proportional to the number of lines of force cut by it, and to the rate of cutting, we may compare the strength of different fields by observing the current resulting from the cutting of them by a wire at e qual speeds. It is advantageous to wind the wire into a small coil and place it in the field with its plane perpendicular to the direction of the lines of force, and then suddenly turn it through a right angle, when, its plane will be parallel to the lines of force, and none will be passing through the coil. The E. M. F. resulting will be proportional to the number of convolutions, the strength of the field and the area of the coil (that is, to the number of lines of force passing through the coil), and to the speed with which the lines are removed. Special galvanometers are constructed to give the values of such sudden momentary currents, by comparing which the strengths of the various fields can be measured.