636 PROCEEDINGS OF THE AMERICAN ACADEMY. 



when it comes into the hands of the observer, and that it is often very 

 difficult, if not impossible, to demagnetize a massive block thoroughly. 

 If a slender rod be placed inside a long solenoid in circuit with the 

 secondary coil of a suitable open-core transformer, and if this coil be 

 slowly drawn off the core with the help of some mechanical device, it 

 is possible to send through the solenoid a long series of currents, alter- 

 nating in direction and gradually decreasing in intensity, and thus to 

 demagnetize the rod well enough for most purposes. The Jefferson 

 Laboratory has three large sets of apparatus of this sort. 



The process just described, however, does not succeed very well with 

 stouter rods, for several seconds may be required to establish a steady 

 current in the solenoid under a steady electromotive force if the core 

 be large, and the use of alternating currents of commercial frequencies 

 is barred out. The solenoid current may be reversed in such a case, 

 at sufficiently long intervals, by means of a mercury commutator geared 

 to an electric motor. Such a commutator, made several years ago by 

 Mr. George W. Thompson, the mechanician of the Jefferson Laboratory, 

 enabled Mr. L. A. Babbitt ® to demagnetize very completely the finely 

 divided core of a large toroidal transformer, though a number of hours 

 were spent each time in the process. With irregular masses of metal 

 this process also is often ineffective, and it is not always successful 

 with short cylinders. A piece of soft Bessemer steel 5 centimeters 

 long, recently cut from a long rod 3 centimeters in diameter, in the 

 Jefferson Laboratory, was found to be slightly magnetized, and Mr. 

 Thompson and Mr. John Coulson attempted to demagnetize it in a 

 solenoid about 38 centimeters long, consisting of about 1460 turns of 

 large wire. They began the series of alternately directed and slowly 

 decreasing currents with one of more than 40 amperes, corresponding 

 to a field within the solenoid before the iron was introduced of about 

 1700 gausses, but the iron was still magnetized in the old direction, 

 with nearly the same intensity, at the end of their work. 



In demagnetizing a stout piece of iron by currents alternating in 

 direction, it is well to put the metal slowly through a succession of 

 complete hysteresis cycles with gradually decreasing ranges, but if this 

 be inconvenient, the iron may be surrounded by a thick copper shell,'' 

 the eddy currents in which will prevent the magnetic changes in the 

 iron caused by a sudden reversal of the main switch from being so vio- 

 lent as they otherwise might be. As will appear more clearly in the 

 sequel, the distribution of the magnetization in a stout iron cylinder 



8 These Proceedings, 47, 1911. 



' Shuddemagen, These Proceedings, 43, 1907. 



