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

 

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.

 

Thomson's Method by a Revolving Coil.

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^[1].

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.

Let	$\gamma$	be the strength of the current in the coil.
	$g$	the total area inclosed by all the windings of the wire.
	$G$	the magnetic force at the centre of the coil due to unit-current.
	$L$	the coefficient of self-induction of the coil.
	$M$	the magnetic moment of the suspended magnet.
	$\theta$	the angle between the plane of the coil and the magnetic meridian.
	$\phi$	the angle between the axis of the suspended magnet and the magnetic meridian
	$A$	the moment of inertia of the suspended magnet.
	$MH\tau$		the coefficient of torsion of the suspension fibre.
	$a$	the azimuth of the magnet when there is no torsion.
	$R$	the resistance of the coil.

1. See Report of the British Association for 1863.