154 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



the number of turns in this secondary was so small and the resistance 

 large, the forms of the curves M, N are not very different from what 

 they would have been if the secondary circuit had been open. The 

 curves F, W were taken with another secondary circuit of 1095 turns 

 closed on itself : the boundary of the area v shows on an arbitrary 

 scale the form of the induced current in this last mentioned secondary 

 circuit. 



It is not to be expected, of course, that a current curve for the ex- 

 citing coil of an electromagnet which has a large solid core will be so 

 much altered in general appearance by the closing of a secondary coil 



ElGOKE 48. 



as it would be if the core were divided so as to prevent in large measure 

 the effects of powerful eddy currents which are present when the iron is 

 in one piece. 



Even in the case of an electromagnet the core of which is built up of 

 broad varnished pieces of sheet iron, eddy currents in this iron may 

 radically change the form of a current curve unless the sheets are very 

 thin. Figure 49 illustrates this fact by an actual example drawn to 

 scale. 



Figure 50 shows curves belonging to a certain transformer. M is a 

 piece of a statical hysteresis curve ; N is a similar curve obtained from 

 a reverse current oscillogram. Although the core of this magnet is 

 made up of varnished pieces of sheet iron, the effects of eddy currents, 

 as will be shown more clearly in the sequel, are here very noticeable. 



Some instances of the phenomenon just mentioned suggest a possible 



