Magnetometer.
(a) A reflecting galvanometer, with heavy magnetic needle, dampened by a
copper frame. It was devised by Weber.
(b) An apparatus for measuring the intensity of magnetic force. It may
consist of a magnet suspended by bifilar or by torsion suspension. A
reflecting mirror and scale as in the reflecting galvanometer may be
used to act as indicator of its motions. It is used in investigations of
the intensity of the earth's field.
If the motions of the spot of light are received on a moving strip of
sensitized paper and are thereby reproduced photographically, the
instrument is self-recording. Such an apparatus is used in the Kew
Observatory, Eng., for recording the terrestrial magnetic elements.
Magnetometry.
The determination of the magnetic moment of a magnet.
It involves the determination by experiment of--( a) the product of the
magnetic moment, M, of the magnet by the horizontal component, H, of the
earth's magnetism; (b) the quotient of M divided by H. Knowing these two
quantities, M is given by the formula M = SquareRoot( )M * H) * (M/H) )
and if desired H is given by the formula H = SquareRoot( (M*H) / (M/H)).
M*H is determined by the method of vibrations. A very long, thin magnet
suspended by a torsion filament is caused to oscillate, and its period
is determined. Calling such period T and the moment of inertia of the
magnet I, we have the formula T= 2* PI * SquareRoot( I / (H*M) ) (a),
whence H*M is calculated, I of course being known or separately
determined.
Fig. 233 END-ON METHOD.
Fig. 234. BROADSIDE METHOD.
M/H is determined by the End-on deflection method, or the Broadside
deflection method. In both cases the deflection of a compass needle by
the magnet in question is the basis of the work.
In the end-on method AB is the magnet under examination; DE the compass
needle; a the angle of deflection; d the distance between C and the
middle of AB, which should be considerable compared with the length of
DE; 2l, the length of AB. We then have the formula
tan a = (M/H) * (2d / (d^2 - l^2)^2),
which if 2l is small compared to d reduces to
tan a = M/Hd 3
(b), which gives M/H, a and d being known.