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PROCEEDINGS uF THE AMERICAN ACADEMY 



circuits of constant inductances, coils containing iron were studied 

 from the point of view of the principles which he had laid down. 

 Helmholtz's own experiments and those of others soon showed, 

 however, that the introduction of masses of magnetic metal into the 

 space within the coils complicated very much their action. It ap- 



FlGTUE 2. 



The curves Q, /.', S represent for different relative values of the mutual in- 

 ductance the current induced in the secondary circuit of a certain induction 

 coil without iron, when the primary circuit is suddenly closed. 



peared that the existence of eddy currents in the iron, if the coil were 

 solid, and the fact that the counter electromotive force in a circuit — 

 as measured hy the time rate of change of the flux of magnetic induc- 

 tion through it — is by no means proportional to the rate of change 

 of the intensity of the current if a circuit "contains iron," made the 

 simple theory of Helmholtz inapplicable, as he himself had foreseen 



