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ID number:  BIRRC-P0001
Institution:  Research and Cultural Collections
Named collection:  Historic Physics Collection
Artist / Maker:  The Cambridge Scientific Instrument Co. Ltd.
Title / Object name:  Earth Inductor
Object type:  Magnetic instrument
Place made:  Cambridge
Date made:  1909
Materials:  Mainly brass, with coil wound on some non-metallic former
Measurements:  20 x 25 x 28 cm

An Earth Inductor is an instrument for determining the direction of the Earth’s magnetic field, in particular the ‘angle of dip’ which is the angle between the field and the vertical. It consists of a coil that can be rotated about its diameter. The coil is connected via a commutator to a galvanometer. When the axis of rotation is adjusted to lie along the Earth’s field there will be no e.m.f. detected by the galvanometer. As this is a null adjustment it can be very sensitive. The instrument has graduated scales with fine adjustment screws to position the axis in both the horizontal and vertical plane. The scales can be read to 1 minute of arc.
The coil is rotated by hand using a detachable handle. Image P0001b is of paper label, possibly original label.

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