If a piece of iron or steel is placed in a magnetic field, it is magnetized by induction. This is true whether the iron or steel is already permanently magnetized or not. Suppose that a permanent magnet is placed in a magnetic field parallel to the direction of the magnet, that is, so that no torque results from the action of the field on the magnet. The strength or magnetic moment of the magnet will be increased (or decreased, if the field of the magnet opposes the applied surrounding field) by induction. The ratio of the change dM, caused by a field of unit-strength, to the original moment M of the magnet is called the induction-coefficient h. The product hM is called the induction-factor of the magnet, designated by μ. It will be seen that the induction-coefficient of a magnet changes with a change of moment of the magnet, being inversely proportional to the moment within the range of field-strengths used in this investigation. The induction-factor μ, for all practical purposes in magnetometer work, may be considered a constant over fairly long periods of time.