ON THE MAGNETIC QUALITIES OF IRON. 
1027 
test. The areas of these curves, which are not affected by this shearing, when 
plotted in relation to B are found to lie on a smooth curve of the tyije drawn 
in fip 21. 
Fiof. 26 is another test of the same class, made with a different curve tracer and 
with other iron. Here the successive cycles are shown in their natural relation to one 
another, without change of zero, and it will be seen that they show well a feature 
which was referred to in the ballistic tests, namely the projection of the extremities 
of each cyclic curve over the rising limb of the cycle next above it. Many other 
examples of curve-tracer tests might be cited as confirming this observation. 
Fig. 27 is closely related to fig. 26. It shows a group of curves taken in immediate 
Fig. 27. 
succession to the group of fig. 26, the iron, the instrument, and the condition of the 
experiment being in every way the same, except that in fig. 27 the cycles were gone 
through comparatively fast—at the rate of 3^ cycles per second—whereas in fig. 26 
the process was quite slow. The rounded corners and increased wideness of these 
curves should be noticed. Their increased area shows that more work is here being 
done on the iron, the effect probably—at least in great part—of Foucault currents. 
The main portion of the magnetic circuit consisted, in this instance, of well laminated 
metal, but thick solid pole pieces were used. 
The curves of fig. 27 were recorded by marking with a pencil the luminous fine 
described upon the screen in the periodic repetition of each cycle. In fig. 28, where 
the results of a group of tests with a different instrument are shown, the curves were 
automatically recorded by allowing the spot of light to fall on a sensitive plate.^ In 
all except the last figure in this group each cycle was gone through in about half 
* tVe are indebted to Mr. A. G. Dew Smith for tliese pbotograpbic records, 
6 P 2 
