The declinometer is an instrument that measures magnetic declination (the direction of the horizontal component of the earth's magnetic field at a given site, with respect to true north). It is used in magnetic observatories and in making magnetic field surveys.

A declinometer contains a cylindrical magnet suspended in a horizontal position by a thin metallic ribbon (usually gold, because of the greater no-torsion stability of gold). In the accompanying picture, the magnet is in the center of the magnet "house" and on the center line of the observing telescope. The magnet, equipped with a mirror on one end, is observed through the telescope.

The instrument is turned in azimuth until the optical axes of the telescope and the magnet are parallel. The angular reading of the precisely divided horizontal circle indicates the direction assumed by the freely suspended magnet. The declinometer is then turned until the telescope is sighted on a distant mark whose true azimuth has already been determined by celestial observations. The difference between the two readings of the horizontal circle is the magnetic declination at the given site.

An absolute declinometer has a provision for inverting the magnet by turning it about its longitudinal axis in order to eliminate errors caused by magnetic or optical imperfections. It has an accuracy of 0.1 minute of arc or better.

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