150 



PROCEEDINGS OF THE AMERICAN 1 ACADEMY. 



meters. The building-up curve of a current in the coil of this magnet 

 is at the beginning slightly modified by the presence of eddy currents 

 in the iron, but the effect is small. When this magnet is put a good 

 many times successively through a cycle with a maximum excitation 

 that will cause a flux density of about 14000, the throws of a ballistic 

 galvanometer used to measure the total flux changes may differ from 

 each other as much as 1% for different rounds, but the averages of a 

 set of measurements made in the first case when the current is put on 

 suddenly and in the second case when it is brought" gradually in per- 

 haps half a minute to its full value are practically indistinguishable. 



Through the kindness of Dr. George Ashley Campbell I have been 

 allowed to use also a number of toroids belonging to the American 

 Telegraph and Telephone Co., the cores of which are made of wire 

 only to of a millimeter in diameter. A hysteresis diagram for 

 any of these coils obtained by a step-by-step ballistic method, agrees 

 exactly, within the limits of my measurements, with a similar diagram 

 obtained by computation from an oscillograph record of a current 

 curve in the coil. As we shall see this fact makes it possible, when 



Figure 1. The electromagnet J, which has a laminated core of square 

 cross-section of about 156 square centimeters area, and is built up of soft iron 

 plates about one third of a millimeter thick. 



