1024 
PROFESSOR J. A. EWING AND MISS H. G. KLAASSEN 
The heating effect of reversals is, of course, due to Foucault currents, as well as to 
magnetic hysteresis. An attempt was made to compare the total heating effect, 
measured directly by reference to the balancing current in the inactive core with the 
value of jH .c/I as found in the preceding experiments, with the view of estimating the 
influence of Foucault currents. But the value of such a comparison depends on our 
being able to estimate the maximum of magnetization in the cycle, and this is by no 
means a simple matter. The observed electromotive force gives the mean value of 
(dB/df)^, and to calculate B from this it is often assumed that the mode of variation 
of B may be treated as simply harmonic, in which case we should have 
Maximum B = 
lO^E 
irn NA v/2 ’ 
where E is the observed electromotive force in a coil of N turns, the area of curve- 
section of the iron being A and the frequency n. But even if the magnetizing force 
were simply harmonic it is otherwise with B, whose increment and decrement is 
largely affected by magnetic hysteresis. The relation of maximum B to mean 
(dB/d^)~ could be determined if the form of the curve of B and t were known. Becent 
experiments—amongst others, those of Dr. John Hopkinson and Messrs. Wilson 
and Lydall,* in which B as well as dB/d^ has been determined as a function of t in 
a ring submitted to alternating niagnetic forces—exemplify how wide the variation 
from a sine-function is likely to be. In view of this consideration, and of the fact 
that no such data were at hand in the case of our own tests, we have not followed 
up this method of directly measuring the heat produced by reversals. 
Experiments made with the Magnetic Curve Tracer. 
When the magnetism of a piece of iron is reversed rapidly more work is spent than 
when the reversal takes place slowly. Whether all the extra work done in the 
former case is to be set down to Foucault currents is, perhaps, an open question ; there 
may be a genuine time-lag in the process of magnetization, independent of the effect 
ot Foucault currents, though not readily distinguishable experimentally from their 
eftect. This quasi-viscosity, distinct from Foucault-current effects, will, if it exists, 
require higlier values of H to be applied than would otherwise be necessary when any 
assigned limits of B are to be quickly reached. At high speeds of reversal the curve 
Vt'hich shows the relation of the externally applied magnetizing force to B becomes 
swelled out and encloses a larger area, corresponding to the greater amount of work 
then done, whether in generating Foucault currents or in overcoming this other 
possible source of quasi-viscous resistance. Tliat an increased amount of energy is 
expended is clear from experiments of the kind which have just been referred to, 
* ‘ Roy. Soc. Proc.,’ 1893. 
