PROPAGATION OF MAGNETIZATION OF IRON. 105 
change of the induction up to that time, which divided by the area of the coil in sq. 
centims. gives the average induction per sq. centim. In obtaining the areas we had 
to assume the path of the electromotive force curve up to 2 seconds, but this we can 
do with a good deal of certainty. 
With regard to the forces we see that after 3 seconds the induced currents have 
to work against a constant current in the copper coils. In obtaining the forces due 
to induced currents we have only taken the area of the curves in fig. 124 between 
the radii 1°22 centims. and 5°08 centims. ; that is, we have neglected the effect of 
the currents within the area of coil No. 1 altogether. The resultant force (H) is 
the algebraic sum of the force (H,) due to the currents between the radii taken, and 
the force (H,) due to the current in the copper coils, and is set forth for different 
epochs in Table II. The inductions per sq. centim. have been plotted in terms of 
this resultant force (H), and curve I., fig. 128, shows this relation. 
Next, take curves II. and III., fig. 128. In obtaining the inductions for these 
curves, the difference between the integrals of curves No. 1 and 2, fig. 12, for a given 
epoch, has been taken. This gives the induction for this epoch, which, when divided 
by the ring-shaped area between coils 1 and 2, gives the average induction per unit 
of that area. 
In obtaining the forces in curve IL., fig. 128, we have taken the areas of the curves in 
fig. 124 between the radii 3:18 centims. and 5°08 centims.; that is, we have neglected 
the forces within the area under consideration as before. Here the error is of more 
importance, and may partly account for the difference between the forces of 
curves I., IJ. In curve III. we have taken the areas of curves in fig. 124 between 
the radii 2°2 and 5:08; that is, we have taken account of the force due to induced 
currents over a considerable portion of the area considered. Coupled with the 
uncertainty in form of the curves in fig. 124 we have the uncertainty as to how 
much to allow for the forces due to induced currents over the particular area 
considered. The difference in the ordinates of curves I. and II. may partly be 
accounted for by errors arising from the assumed path of the electromotive force 
curve up to 2 seconds, which is more uncertain in curve 2, fig. 12, than in curve 1 ; 
and partly to possible slight inequality between the muterials of the rod and its 
surrounding tube. 
In fig. 13 the maximum current in the copper coils is ‘77 ampere, corresponding 
with a force in C.G.S. units of 16. The current in the copper coils, after passing 
through zero, attains its full value at about 9 seconds after reversal, and the change 
of induction ceases at 19 seconds. 
No. 1 curve, fig. 13, has been integrated, and the maximum induction per 
sq. centim. found to be 14,500 C.G.S. units. We have taken a given cyclic curve 
for soft iron corresponding with this maximum induction, and have tabulated the 
forces obtained therefrom in Table ILI. for the different values of B got from the 
integration of No. 1 curve. We then plotted in fig. 134 the amperes per sq. centim, 
MDOCCCXCV.-—-A. P 
