Magnetograph
an instrument for recording the fluctuation of the magnetic field of the earth. The variations in strength only are recorded-the exact value at any given time is usually obtained by use of a magnetometer. The principle of operation most commonly used is the reflection of a ray of a light from a mirror attached to a suspended or freely moving magnetic needle to a recording sheet of photographic paper which is attached to a drum actuated at a uniform speed by clockwork. When the instrument is to be used for recording the magnetic declination, the magnetic needle is usually suspended by a quartz fiber. When used to measure the vertical component, the needle is mounted on a knife edge so that its axis about which it may move is horizontal.
Any movements of the needle, which should be of light weight in order that the inertia may be of a minimum, will cause a movement of the image reflected upon the recording paper, and a curve representing the relative changes of the magnetic is obtained.