ON THE MAGNETIC QUALITIES OF IRON. 
1033 
movements of tlie mirror of the curve traces take place as if it were controlled by a 
dash-pot, or were immersed in a viscous fluid. Even a comparative!}’’ slow gradual 
change in the magnetizing current is followed, after it ceases, by a continued creeping 
of the reflected spot of light lasting for some seconds,^ and, when a cycle of reversal is 
performed at even a very moderate pace, the curve assumes a wholly different form 
from that which would correspond to a quite slow process. This is illustrated by 
fig, 33, where {a) is the curve taken quite slowly for a pair of soft iron rods 
1'9 centim. in diameter, and (6) and (c) show the results when the period of a cycle 
was 3 seconds and 0*43 second respectively. 
The influence which even the lower of these speeds has on the form of the curve is 
extraordinary. The maximum of the magnetizing current is nearly the same in all 
three curves (owing to the self-induction of the magnetizing coil it is a little less in 
the faster cycles, but enough non-inductive resistance was put in the circuit to make 
this difference small). It will be seen that the effect of time-lag in lowering the ' 
maximum of magnetization (namely, the maximum of the mean magnetization taken 
over the cross-section of the rods) is enormous; the effect of change in frequency 
being most noticeable when the period is comparatively long. Again, the influence of 
time-lag in augmenting the work spent in magnetic reversals is conspicuous, and here, 
again, it is remarkable how great this influence is at very moderate speeds of reversal. 
The exact form which the curve takes at any assigned speed will, of course, depend 
on what particular function of the time the applied electromotive force is caused to be, 
as a consequence of the exact form of the commutator plates and other conditions of 
the experiment. In the present case it appeared that the applied electromotive 
force w’as made to vary lather quicker near its mean values and rather less quick 
near its extreme values than would have been the case had it been a simple harmonic 
function of the time. 
It is noticeable that the area of the curve increases at first and finally decreases as 
the frequency is increased,. In other words, with a given maximum of magnetizing 
current the work expended in reversals is a maximum, per cycle, for some one 
frequency. This, of course, results from the fact that at higher frequencies the 
magnetization fails to penetrate suflSciently into the interior of the bar. 
In steel these phenomena are much less apparent than in soft iron, but they take 
place to some extent. Fig. 34 shows the results of a similar experiment made with 
a pair of bars of tool steel of the same dimensions as the iron rods of fig. 33. Here 
again {a) shows the effects of perfectly slow reversal, while {h) and (c) are cycles 
performed in periods of 3 seconds and 0'6 second respectively. In both of these 
experiments (figs. 33 and 34) the cycles marked (6) and (c) were produced by using 
an electric motor to drive the commutator, regularity of speed being essential to 
steadiness of the figure, especially at the slower speeds. 
Whether these manifestations of time-lag in magnetization are entirely explicable 
* Compare the direct magnetometric observations described in ‘ Roy, Soc. Proc.’ June 20, 1889. 
MDCCCXCIII.—A. 6 Q 
