112 MESSRS. J HOPKINSON AND E. WILSON ON THE 
The method of experiment was as follows :—The liquid reverser, M, was placed so 
as to give a maximum current on the meter G, which was adjusted by non-inductive 
resistance, N, to the desired value, and, in all cases, when changing from higher to 
lower currents, a system of demagnetization by reversals was adopted. Time was 
taken, as in Part I., on a clock beating seconds, which could be heard distinctly. 
As an example, take fig. 19, in which the periodic time is 80 seconds, and the 
maximum current in the copper coils °23 ampere. The E.M.F. curves of the exploring 
coils are numbered 1, 2, and 3 respectively, and the curve of current in the copper 
coils is also given. 
As in the case of simple reversals (Part I.) we may from these curves attempt to 
obtain an approximation to the cyclic curve of induction of the iron. In all cases 
where this is done we have taken coil 1 and considered the area. within it—that is to 
say, from a knowledge of the E.M.F.’s at different depths of the iron, due to change 
of induction at any epoch, we have estimated the average magnetizing force acting in 
this area, and this we cali H,. The curves from which these forces have been 
obtained are given in fig. 194, and have been plotted from Table VI. The algebraic 
sum of this force, H,, and the force H,, given at the same epoch by the current in 
the copper coils, is taken to be the then resultant force magnetizing this area. Also 
the integral of curve 1, fig. 19, gives the average induction over this area at the same 
epoch. Curve , fig. 19B, is the cyclic curve obtained by plotting the inductions in 
terms of the resultant force H. 
A word is necessary with regard to the last column in Table VI. This gives the 
total dissipation of energy by induced currents in ergs per cycle per cub. centim. of 
the iron. We know the watts per sq. centim. at different depths of the iron for 
different epochs. Let a series of curves be drawn (fig. 19) for chosen epochs giving 
this relation: the areas of these curves from radii 0 to 5:08 give for the respective 
epochs the watts per centim. dissipated by induced currents. In symbols this is 
— dy; where r is the radius, and ec the E.M.F. and current. It is now only 
Sq. centim. : 
necessary to integrate with regard to time in order to obtain the total dissipation : we 
éc 
have chosen a half period as our limits. This gives us || dr dt, and is got 
from the area of curve z, fig. 19p. The ordinates of this curve are taken from the 
last column of Table VI. 
The curves in figs. 21, 22, have been treated in a similar manner to that already 
described in connection with fig. 19. But in fig. 20 the procedure is a little different. 
In this case the periodic time is 20, and the maximum force per centim. linear, due to 
the current in the copper coils, is 4°87. With this frequency and current the effects 
of induced currents in the iron are very marked: we have taken a given soft 
iron cyclic curve, of roughly the same maximum induction as given by the integral of 
curve No. 1, fig. 20, and have tabulated the forces obtained therefrom in Table VII, 
