104 MESSRS J. HOPKINSON AND E. WILSON ON THE 
Table I. gives a list of the experiments made with total reversal of current due te 
54 cells, the magnetizing coils being kept in parallel with one another, and the 
- magnitude of current through them adjusted by means of a non-inductive resistance. 
In fig. 10 the maximum current in the copper coils is ‘0745 ampere, which, after 
reversal, passes through zero and attains a maximum at about 3 seconds. It will be 
observed that the change of induction with regard to each of the coils 1, 2, 3 is 
rapid to begin with, but that it gradually decays and becomes zero at about 46 seconds 
after reversal. 
Fig. 11 is interesting in that it gives the particular force at which coils 1 and 2 
show a second rise in the electromotive force curves, No. 1 being a maximum at 
about 25 seconds, and No. 2 at about 8 seconds after reversal. These “humps” 
become a flat on the curve for a little smaller force, and, as shown in fig. 10, 
they have disappeared altogether. In this case the current in the copper coils has 
attained a maximum at about 4 seconds after reversal. 
In fig. 12 the maximum current in the copper coils is ‘24 ampere, corresponding 
with a force in C.G.S. units of 4:96. This is got from = es The current 
in the copper coils has attained its maximum value at about 4 seconds after 
reversal, and changes of induction were going on up to 35 seconds. 
In the following attempt to obtain an approximation to the cyclic curve of 
hysteresis, from these curves, we bave taken the volume-specific resistance of the 
soft steel to be 13 X 107° ohm. We have taken the diameter of coils 1, 2, 3 to be 
respectively 1°22, 3°18, and 5:08 centims.,* and we find that the corresponding 
resistances, in ohms, of rings of the steel having 1 sq. centim. cross-section and 
mean diameters equal to the coils are, respectively, 103°7 x 107°, 259°4 x 107%, and 
4164X10~°. From a knowledge of the electromotive forces at the three radu, for 
a given epoch, we are able to find the amperes per sq. centim. at those radii. In fig. 
124 a series of curves have been drawn for different epochs, giving the relation 
between amperes per sq. centim. and radii in centims., and the areas of these curves 
between different limits have been found, and are tabulated in Table II. It is 
necessary here to state that the path of these curves through the four given points 
in each case is assumed ; we have simply drawn a fair curve through the points. 
But what we wish to shew is that the results obtained with the curves, drawn as 
shown in fig. 12, are not inconsistent with what we know with great probability to 
be true. 
The results shown in fig. 12B have been obtained as follows: take curve ], 
fig. 128; the electromotive force curve of coil 1, fig. 12, has been integrated, 
and the integral up to the ordinate corresponding to any time is equal to the total 
* Tn Part IJ. of this paper the smallest radius was taken to be 127. cr our purpose the difference 
is not worth the expense of correction. 
