122 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



crease in intensity to zero. One common way of doing this is to 

 attach the coil to the secondary of a sufficiently powerful alternate 

 current transformer so arranged that the primary coil may be slowly 

 withdrawn to a long distance from the secondary. In the case of the 

 soft iron wire the demagnetization is sometimes accomplished by 

 heating the wire red hot. 



It is often a matter of considerable difficulty to remove entirely the 

 effects of previous magnetization from the completely closed massive 

 core of a large transformer : even if the source of a current in the 

 exciting coil has a high voltage, several seconds may be required to 

 established the current, and the use of an alternating demagnetizing 

 current in the coil, with any commercial frequency, is barred out. If 

 a powerful storage battery be connected to the exciting coil through a 

 commutator and a suitable "liquid rheostat," one may begin with a 

 sufficiently strong current (Iq) and, after reversing this several times 



Figure 16. 



Forms of ,the cross-sections of the laminated cores of tlie electromagnets 

 Q and R. 



by hand, increase a little the rheostat resistance so as to decrease the 

 current slightly, then reverse this weaker current a number of times, 

 and thus proceed until the current is reduced to a very small value ; 

 but if the core is very large, the operation may take a couple of hours 

 even if the number of steps is not excessive, and after all, it is not 

 easy to teU whether the work has been successful. If the initial 

 current was strong enough, if the stages were sufficiently numerous 

 and properly spaced, and if the number of reversals at each step was 

 great, one may, of course, expect to find the core pretty thoroughly 

 demagnetized, but to test the matter it is usually necessary to undo 

 what has been accomplished by determining the amount of magnetic 

 flux sent through the core when a current of given intensity (7) is sent 

 through the exciting coil. This amount ought to be the same whether 

 this testing current has the same direction as that of the last applica- 

 tion of the large current (I^) or the opposite direction, and unless one 



