﻿Magnetizing and Astaticizing Galvanometer-Needles. 483 



the process of hardening and tempering or subsequently*. 

 During the process of building up, the individual magnets 

 are subjected to various demagnetizing influences, the most 

 serious of which are the jars and blows received in the pro- 

 cess of mounting, and the proximity of other magnets during 

 the process of astaticizing. The resultant strength of each 

 member of the finished system is therefore considerably less 

 than it would be if the individual magnets were magnetized 

 to their saturation-point. Prof. Threlfall | avoided this diffi- 

 culty by magnetizing each member of the system in situ, 

 using for this purpose two separate electromagnets, one for 

 each member. 



It was this which first suggested to me the present method, 

 which differs from the preceding in that but one electro- 

 magnet is used for simultaneously magnetizing both members 

 of the system. The advantage of this is that both members, 

 when being magnetized, form part of one and the same 

 magnetic circuit, and hence, barring magnetic leakage (which 

 may be reduced to a minimum by proper design of circuit), 

 are necessarily in fields of precisely the same strength for all 

 magnetizing forces — one essential condition for securing 

 astaticisin, or in preserving it when once established and 

 remagnetization is desirable. When separate electromagnets 

 are used for each member, it is necessary in order to secure 

 this condition that the field of each be of exactly the same 

 strength, an end which requires for its accomplishment 

 the taking of unusual precautions (described in the paper 

 referred to) in the selection of material, the winding of the 

 coils, &c. The single electromagnet device is therefore con- 

 siderably simpler to construct and more convenient to use. 



r; 



g 



Fig. 1 is an elevation of the electromagnet in the form in 



* Simultaneous magnetization and hardening does not seem to possess any 

 advantages over the ordinary method. See Holz, Wied. Ann. vii. (1879). 

 t " Measurement of High Resistance," Phil. Mae-, vol. xxviii. p. 452 

 (1889), 



