Riding the wave

Wild's Earth Inductor

The instrument provides a measurement of magnetic inclination, based on the electromotive forces that a rotating coil immersed in a magnetic field generates. The earth inductor is therefore basically a small generator that can provide the measurement once immersed in the earth's magnetic field.

The core of the instrument is a coil inside which is placed a level. The coil rotates around a diametral axle. The axle is resting on two agate bearings set in a circular frame, the latter being rotating around a horizontal axis. The two bearings that support the axle are, in turn, firmly fastened to a horizontal ring, that, like the alidade of a theodolite, can turn on a horizontal graduated ring. The brass scale 0°-360° is marked every 10° and is divided into degrees. By means of this ring the coil can take any azimuth. The azimuth ring is engraved with the name of manufacturer Edelmann München.

Sideways and perpendicular to the coil frame, a vertical ring can be read by two microscopes; on the left a micrometer screw for fine adjustment of coil axis angle allows to get the minute, but naked eye also a half degree can be appreciated. The vertical ring has a diameter of 13 cm and its microscopes allow a reading of tenths of a minute, its scale is silver plated. The coil has a diameter of 10 cm and the horizontal ring 24.8 cm.

The coil was connected to a Thomson type galvanometer, that is lost, and also lost are the compass and the level, that was placed across the coil axle. The flexible cable and the handle for connecting the coil and the galvanometer are a remake.

(Taken from Two centuries of geomagnetic instruments in Italy (1740-1791) , 1997)
Wild's Earth Inductor
By Paola Presciuttini
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