0 ^ THE MAGNETIC QUALITIES OF IRON. 
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from zero, after the process of demagnetizing by reversals had been performed. This 
line will be seen to pass through the extreme points of the several cyclic curves, which 
thus overlap one another in the manner already described. 
Fig, 31 is a study in superposed magnetizations in soft iron. Here the magnetic 
curve tracer was arranged with two independent magnetizing coils upon the magnetic 
cn-cuit under examination. One of these was used to apply a cyclic variation 
of magnetic force, while the other was used to apply a steady force of greater or less 
intensity, on which the cyclic force could be superposed. For brevity, we may 
distinguish the two circuits as P and Q. In the first place, after the iron had been 
demagnetized by reversals, the current in Q was applied, and was reversed repeatedly 
between equal positive and negative values, there being, at the time, no current in P. 
This gave the usual cyclic curve ah (fig. 31), from which it will be seen that the 
magnetizing force then applied by Q was comparatively small. Then (the piece having 
been again demagnetized) a current was set up in P, which brought the magnetism up to 
a value represented by the point c in the figure. This current in P being now kept 
