A form of Hughes' magnetic balance, which is useful for rapidly
determining the relative magnetic qualities of two or more samples of
matericil, is shown in Fig. 225. It consists of a long bench or table
supported on two standards on a base fitted with levelling screws. At
one end (the right-hand) of this raised bench is a magnetizing solenoid,




Fig. 225.
in which is inserted the specimen to be tested, and at the other a small
balancing solenoid to neutralize the inductive effect of the magnetizing
coil by itself on a magnetometer needle suspended between the two
coils. A permanent steel compensating magnet (seen to the extreme
left) can be turned round a pivot passing through the centre of a
vertical scale fixed to the base, and its deflections made to neutralize
the various stages of magnetization of the specimen itself The
method is a "zero" one, and the indications or position of the
magnetometer needle is read by forming an image of a vertical pin,
fixed just in front of the needle box, in the moving plane mirror
attached to the needle, which is half covered by a fixed plane mirror
immediately in front of it. Thus the zero position of the needle is



356



Apparatus.
when the two images coincide. Professor S. P. Thompson has shown
that, when the distance from needle to centre of compensating magnet
is 2 '3 times the lattcr's length, the angle through which the magnet
is turned (up to 6o°) is proportional to the magnetic force due to the
iron core or specimen in the magnetizing coil.
Hughes' Induction Balance.
