The terrestrial magnetic field will in general be disturbed in the neighbourhood of an electric circuit; but if the circuit be a plane set at right angles to the terrestrial lines of force, the direction of the field will remain unchanged at all points in the plane of the circuit. To determine the magnetic meridian, we have only to place a plane circuit in such a direction that, when a current in the circuit is started and stopped, no change takes place in the position of a small magnet hung at a point in the plane of the circuit, and free to turn in azimuth. The plane of the circuit will then lie magnetically east and west.