124 



PROCEEDINGS (iF THE AMERICAN ACADEMY. 



superposed on a screen, whether the current had the same direction as 

 its predecessor or the opposite direction. 



If the core of an electromagnet happens to be a straight bar, or a 

 straight bundle of wire, it may be demagnetized by a long series of 

 currents which have alternately one direction and the other, and which 

 slowly decrease in intensity from an initial value which may be con- 

 siderably smaller than the current which magnetized the iron. Figure 

 17 shows the results of experiments upon a rod of soft steel 80 diame- 

 ters long in a long solenoid. The arrangement of the apparatus is 

 shown in Figure 18. The extreme value of the magnetizing field was 

 _'7 gausses, and the average moment per cubic centimeter which the 



field caused was 246. At the outset the core was thoroughly demag- 

 netized, then a series of steady currents, each a little stronger than the 

 last, was sent through the coil, and the moment of the rod was deter- 

 mined for each direction of the current. This gave the curve WJTOQ I". 

 Then the hysteresis diagram I r GKWMZ I ' was obtained, and after the 

 core had returned to the condition indicated by the point V, the 

 current was somewhat decreased until the core " reached " the point H, 

 and then this current was reversed in direction one hundred times, 

 after which (when the current had the positive direction) the iron had 

 exactly arrived at the point on the curve ()/(/ V beneath />'. The core 

 was then brought to V again, the current was decreased, — this time 

 until the core reached the point P, — this current was reversed one 

 hundred times, and it was then found that when it ran in positive 



