ON THE MAGNETIC QUALITIES OF IRON. 
1025 
even if the exact interpretation of these results is somewhat doubtful through un¬ 
certainty as to the value of B. Direct evidence of the change in form of the B—H, 
or rather B-current, curve is furnished by the magnetic curve-tracer. 
This instrument, which was exhibited by one of us at the Edinburgh meeting of 
the British Association (1892), draws the magnetic curve by giving to a small mirror 
two component motions, one (horizontal) proportional to the magnetizing current and 
the other (vertical) proportional to the magnetism of the metal under examination. 
The metal is arranged to form a magnetic circuit which is complete except for a 
narrow gap between a pair of opposed poles. In this gap a wire is tightly stretched, 
through which a constant current passes. The wire consequently sags up or down in 
the gap as the magnetism of the circuit varies, and this gives movement in altitude 
to the mirror. At the same time, the varying magnetizing current passes through 
another similar wire tightly stretched between the poles of another magnet. The 
magnetism of this second magnet is kept constant, a,nd hence the sagging of its wire 
depends on the varying strength of the current which the wire carries, namely, the 
magnetizing current of the other mtignet. This gives azimuthal movement to the same 
mirror. A spot of light reflected from the mirror to the screen consequently traces 
out the curve of magnetism and magnetizing current. The movements are so dead¬ 
beat that with a light mirror and tightly stretched wire a magnetic cycle of double 
reversal may be traversed ten times or even twenty times per second without 
material distortion from the'"inertia of the working parts. The variation of mag¬ 
netizing current must not, however, be sudden, and to secure this a commutator is 
used in which a revolving boss carrying two zinc plates turns within a cylinder con¬ 
taining a weak solution of zinc sulphate, in which there are two other (fixed) plates 
of zinc. As the revolving pair of j^lates reverse their position between the fixed pair, 
the potential of a battery connected to the fixed plate becomes reversed on the 
revolving pair, and from them the current is taken to the magnetizing coil.* 
By the slow manipulation of a rheostat of any form the magnetizing force may be 
cai'ried throughout any desired series of changes and the character of the corre¬ 
sponding process of magnetization be studied. The instrument affords a ready means 
of testing the comparative magnetic quality of given samples of metal. It also allows 
features in the magnetizing process to be examined which it would be difficult to 
observe in other ways. We have made a number of experiments with it, some of 
which may be cited as showing the application of a novel method in magnetic 
research, and also as bearing incidentally on points that have been ah'eady spoken 
of in connection with the ballistic tests. 
Fig. 25 gives the curves drawn by the magnetic curve tracer in a series of tests of 
a pair of bars built up of soft thin sheet iron. Four cycles are shown, the magnetizing 
current having had as extreme values three, two, one and one-half amperes respec- 
* lor diagrams and description of the magnetic curve tracer, see the ‘ Electrician ’ of May 26, 1893, 
MDCCCXCm.—A 6 P 
