408 Mr. G. Chrystal on Bi- and Unilateral 



I. An elongated magnet magnetized axially would give phe- 

 nomena analogous to those observed. 



II. A spherical magnet would give no such phenomena. 



III. An elongated magnet magnetized transversely would 

 give similar phenomena, except that in unilateral deflection 

 the sign of the effect would be reversed ; i. e., the spot of light 

 being brought, by means of the deflecting magnet, to the right 

 or left of zero, the effect of the alternating currents would be 

 to diminish this deflection. 



Conclusion III. was directly verified ; and the result was in 

 complete agreement with theory. Conclusion II. w T as also 

 verified experimentally. A small spherical steel magnet was 

 fitted with a mirror hung up in the galvanometer and observed, 

 as will be afterwards described in the case of an elongated 

 magnet magnetized axially. The .magnetic moment of the 

 sphere was roughly determined for me by Mr. Shaw, B.A., of 

 Emanuel College ; the maximum horizontal earth-couple on the 



sphere was about '22 grm.f — : ) 



Two cells of Grove were used in the primary of the induc- 

 tion-coil. The result was a feeble unilateral deflection. 

 An observation was made with an elongated magnet con- 

 sisting of a piece of thin watch-spring magnetized longitudi- 

 nally. The maximum earth's couple in this case was about 



'21 grm. I- — ) . All the other arrangements were exactly a 



before. The result was strong hilateral deflection. Here the two 

 magnets were very nearly in the same circumstances, the ad- 

 vantage being somewhat in favour of the sphere, owing to its 

 smaller moment. It appears, then, that the form of the mag- 

 net has a very powerful effect on the phenomenon. 



That the effect should be absolutely nil with the sphere was 

 not to be expected ; for we know * that a piece of steel once 

 permanently magnetized by a force F is never in the same 

 state as it was originally. We can demagnetize the steel ap- 

 parently completely by a force less than F ; but it requires a 

 force greater than F to magnetize to an equal degree in the 

 opposite direction. The particles of a magnetized steel sphere 

 have therefore a <7'w<m'-crystalline structure related to the mag- 

 netic axis ; so that the perfect symmetry which causes a sphere 

 to behave neutrally in a field of uniform force, as far as induced 

 magnetism is concerned, is probably in some degree lost, and 

 can only be restored by heating the steel over red heat and 



* E. y. Rowland, Phil. Mag. [IV.] vol. 1. p. 358. Also Frornrne, Pogg. 

 Ann. Ergbd. vii. p. 421 ; Phil. Mag. [IV.] vol. 1. p. 299. 



