no 



PROCEEDINGS OF THE AMERICAN ACADEMY. 





dt. 



(11) 



If an abscissa I centimeters long corresponds to one second, and an 

 ordinate m centimeters represents one ampere, and if Ao,i stands for 

 the area in square centimeters bounded by the current curve, the 

 asymptote, and ordinates corresponding to the times to, ti, the change 

 in the flux of magnetic induction through the circuit during this time- 

 interval is (in maxwells) 



Im 



(12) 



If, after a current has been built up by stages in the coil of an 

 •electromagnet, in the manner indicated by curve U of Figure 4, the 



SECONDS. 



The shaded area represents on a certain scale the change in the flux of mag- 

 netic induction tlirough a circuit when the resistance of the circuit is suddenly 

 increased and then kept constant. 



process be reversed, and the resistance of the circuit be increased by 

 steps, the current curve wiU look very much as the curve U would if 

 looked at from the wrong side of the paper when upside down. 



As has already been stated, it is possible to get slightly different 

 hysteresis diagrams for a massive core originally demagnetized, when 

 the current is made to change from a given positive limit to the 

 negative limit in different ways ; and it is important, in predicting the 

 behavior of a magnet which is to be used for a given purpose, to 

 employ in computation the hysteresis diagram which corresponds to 

 the particular magnetic journey which the core will take in practice. 

 A single carefully made curve of the U tj^e with a dozen steps will, 

 however, give a result good enough for any commercial purpose, though 



