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8 - Cambridge earth inductor



The Cambridge Earth inductor measures the dip I of the Earth magnetic field.

Induction inclinometers (also called inductometers) were introduced at the beginning of the century, and they progressively replaced the dip compass which was not sufficiently reliable.
The inclinometer on view was made by the English firm, Cambridge Instrument Company, which developed this model after the second International Polar Year (1932-1933). The apparatus on view was obtained in 1956 for International Geophysical Year and had a Broca galvanometer with a portable magnet. This set was used in Port-aux-Français obsevatory (Kerguelen islands) between 1957 and 1962. An identical Cambridge inclinometer was the reference apparatus of Chambon la Forêt French magnetic observatory until 1986.

Inclinomètre profil

Inclinomètre détail

The complete set of apparatus consists of the actual inclinometer, a commutation box and an astatic galvanometer.

The Cambridge Earth inductor is made of a flat coil in continuous rotation around an axis parallel to the spires. When the coil's rotation axis is not confused with the direction of the Earth magnetic field F, the result is an electromotive induction force at the edges of the coil which can be detected in an exterior circuit using a galvanometer. The magnetic flux through the coil is cancelled when the coil's rotation axis is made to follow the direction of the magnetic field F. For mechanical and symetric reasons, the "coil" is in fact two flat parallel coils connected in series a nd placed at each end of the rotation axis. The dip is read using a microscope with an optical micrometer graduated in seconds. In the measurement of dip I, the precision obtained is of a few arc seconds. Introduced since the beginning of the century, induction inclinometers (or inclinometers) progressively replaced the less precise dip compass.

Induction method

The inclination is measured using an induction method. The extremities of the detecting coil, kept in rotation during the measurements, are connected electronically to a ring attached to the axis and divided into four sectors. The exterior circuit has brushes which rub against the ring: the electromotive force at the extremities of the circuit, detected by two rubbers at 180°, is cancelled out when the coil's rotation axis is influenced by the magnetic field vector. A commuter allows the observer to plug in the wires from the four sectors two at a time on an astatic galvanometer. The commutation in DEC position allows the orientation fault to appear in azimuth (the vertical circle must first be brought into the magnetic meridian, and then the instrument is oriented in azimuth). The dip inclination is then found by commutation in INC position : the coil's axis is progressively dipped until the electromotive force, detected by the galvanometer and visualised by the displacement of the spot on the graduated ruler, is cancelled. The dip's value is read on the graduated vertical circle (diameter : 203mm) with a precision of a few arc seconds.