PEIRCE. — BEHAVIOR OF THE CORE OF AN ELECTROMAGNET. 123 



has a hysteresis diagram for the core obtained by using currents which 

 range exactly between +/(, and — /„ the whole work must be done twice. 

 The determination of the flux changes may be made very conveniently 

 with the help of a fluxmeter, but if the highest accuracy is required, 

 it is better to take an oscillogram of the building-up curves of the 

 current when the core starts from its state of supposed neutrality. 



If the core of a large electromagnet is not quite closed, it is compara- 

 tively easy to demagnetize the iron almost completely and to prove 

 that this has been done ; indeed, if the gap has the proper width, the 



Figure 17. 



iron practically demagnetizes itself in a wonderful manner. An in- 

 stance of this was given by Professor Thomas Gray in the case of a 

 40 K. W. transformer, and I found that the hysteresis diagram for a 

 certain electromagnet which has a solid core the area of which in its 

 slenderest part is more than 450 square centimeters, consists prac- 

 tically of a single straight line when the air gap has a width of 35 

 millimeters. With this magnet, using an excitation of either 7800 

 ampere-turns or 15,800 ampere-turns, I obtained current-time curves 

 which were wholly indistinguishable even when much enlarged and 



