Magnetometer is, in general, any instrument for measuring magnetic force, or for comparing one magnetic force with another. A freely-suspended magnet, whereby the strength and direction of the lines of force in a magnetic field may be ascertained by observing the position assumed by a freely-suspended magnet and also its rate of oscillation and the amount to which it is deflected when under the influence of a second magnet, is the essential feature of all magnetometric instruments. The peculiar importance to us of the earth's magnetic field has, however, led to the construction of instruments of precision, to which the name Magnetometer is specially applied.
In a magnetic observatory the self-registering magnetometers or magnetographs form an extremely important set of instruments. By these the quick changes in the intensity of the earth's magnetic field and in the declination are registered by photographic means. The essential feature of the method is the reflection of a beam of light from a mirror attached to a magnet, which is suspended or pivoted so as to be sensitive to changes in the particular element that is being measured.