CONTRIBUTIONS FROM THE JEFFERSON PHYSICAL 

 LABORATORY, HARVARD UNIVERSITY. 



THE EFFECTS OF SUDDEN CHANGES IN THE INDUC- 

 TANCES OF ELECTRIC CIRCUITS AS ILLUSTRATIVE OF 

 THE ABSENCE OF MAGNETIC LAG AND OF THE VON 

 WALTENHOFEN PHENOMENON IN FINELY DIVIDED 

 CORES. CERTAIN MECHANICAL ANALOGIES OF THE 

 ELECTRICAL PROBLEMS. 



By B. Osgood Peirce. 



Presented May 11, 1910. Received December 31, 1910. 



In making some kinds of electrical measurements, one occasionally 

 needs to alter abruptly the inductances of a circuit and to inquire what 

 the effect of the change is upon the march of the currents which the 

 circuit is carrying. If the circuit happens to be a simple one with no 

 magnetic metals and no other circuits near, and if the whole change 

 takes place in a sufficiently short time, it is easy to compute the mag- 

 nitude and the direction of the corresponding change in the current. 

 If, however, the circuit is complex, or affected by the presence of other 

 inductive circuits in the neighborhood, and if the duration of the 

 change in inductance is long compared with the various time constants 

 which enter, the problem may be much more difficult ; though if there 

 be no magnetic metals in the field, the principles laid down more than 

 forty years ago by Maxwell ^ in his dynamical theory of the electro- 

 magnetic field, and soon afterwards elaborated and illustrated by 

 Rayleigh and others, point the way to the solution. 



In most cases which present themselves in practice, there are masses 

 of magnetizable metal in the form of cores, near the circuits to be 

 studied, and it is often difficult, even if one knows something about 

 the magnetic properties and the history of the cores, to predict exactly 

 what the effects of a given sudden change in the inductances will be. 



^ Maxwell, Philosophical Transactions, Dec. 1864; Rayleigh, Philosophi- 

 cal Magazine, 38, 1869, 39, 1870, 30, 1890. 



