PEIRCE. 



CHANGES IN INDUCTANCES OF ELECTRIC CIRCUITS. 



581 



dotted line which rises at about 1.1 seconds after the circuit was closed, 

 and is an exact copy of the curve D moved to the left. This curve 

 coincides with C for a large part of its course, but has a trifle less area 

 above it than that portion of C has which lies to the right of the ordi- 

 nate at which the lowest part of the dotted curve begins. The shape 

 of D just at the beginning points to the existence of eddy currents. 



P M SECONDS. 



Figure 37. Current curves for a toroid with fine wire core. The second 

 part of a two-stage current is exactly the same as if the current were allowed 

 to grow at once to its final value. 



To test more thoroughly the effect upon the flux of magnetic induc- 

 tion through the core of the transformer, of building up the current in 

 different ways, I first measured with great care, by aid of a modified 

 Rubens-du Bois "Panzer Galvanometer," the changes of this flux for a 

 quick reversal of an excitation of 1812 ampere turns. I then measured 

 by means of the planimeter a long series of oscillograph records obtained 

 by reversing the same excitation by a considerable number of steps. 

 All the testing instruments were different in the two cases, and no 

 comparison was possible until the final results were reached and were 

 found to difi'er from one another by only one part in fourteen hundred. 

 The labor of reducing the oscillograms was so great that this close 

 agreement must be considered accidental, but there can be little doubt, 

 I think, that the flux change due to the single step and the sum of the 

 changes due to the long series of steps which together cover the same 

 change of excitation were practically indistinguishable. 



Figure 37 shows copies of oscillograms taken with a number of 

 toroids in series. The core of each toroid was made of perhaps fifteen 



