Page:A Treatise on Electricity and Magnetism - Volume 2.djvu/396

364 The determination of the value of $$m$$, the magnetic moment of the suspended magnet, is the most difficult part of the investigation, because it is affected by temperature, by the earth s magnetic force, and by mechanical violence, so that great care must be taken to measure this quantity when the magnet is in the very same circumstances as when it is vibrating.

The second term of $$R$$, that which involves $$L$$, is of less importance, as it is generally small compared with the first term. The value of $$L$$ may be determined either by calculation from the known form of the coil, or by an experiment on the extra-current of induction. See Art. 756.

763.] This method was suggested by Thomson to the Committee of the British Association on Electrical Standards, and the experiment was made by M. M. Balfour Stewart, Fleeming Jenkin, and the author in 1863.

A circular coil is made to revolve with uniform velocity about a vertical axis. A small magnet is suspended by a silk fibre at the centre of the coil. An electric current is induced in the coil by the earth's magnetism, and also by the suspended magnet. This current is periodic, flowing in opposite directions through the wire of the coil during different parts of each revolution, but the effect of the current on the suspended magnet is to produce a deflexion from the magnetic meridian in the direction of the rotation of the coil.

764.] Let $$H$$ be the horizontal component of the earth's magnetism.