RuTHEitFORD. — On the Magnetization of Iron. 489 



There was also a marked difference between the effects 

 produced by discharges in opposite directions on the same 

 saturated needle. 



(1.) When the first half-oscillation tended to magnetize the 

 needle in the same direction as it already was magnetized 

 the first half-oscillation had no effect on the needle, it being 

 already saturated. The second half-oscillation tended to de- 

 magnetize the needle, the third to magnetize it, and so on. 



As an example of the effect of continued sparks in this 

 direction we have the following : — 



No. of Sparks. Steady Deflection. 



... ... ... ... 252 



1 ... ... ... ... 226 



2 ... ... ... ... 217 



Several sparks ... ... ... 210 



(2.) When first half-oscillation tended to demagnetize the 

 needle the effect on the reduction of the deflection is much 

 greater ; for example : — 



No. of Sparks. Steady Deflection. 



... ... ... ... '252 



1 ... ... ... ... 156 



2 ... ... ... ... 102 



3 ... ... ... ... 83 



4 ... ... ... ... 67 



5 ... ... 54 



6 ... ... .. ... 45 



J ... ... ... ... Ad 



10 ... ... ... ... 15 



20 sparks ... ... ... 10 



The deflection did not fall below 10 div., however many 

 sparks were passed. The iron has then arrived at the steady 

 state. The gradual demagnetization of iron by successive 

 discharges is well illustrated by the above table. The cause 

 of the effect was not at first clear, but further experiment 

 showed that it was due to the very rapid damping of the 

 oscillations. The first oscillation demagnetizes the surface- 

 layers, and probably magnetizes a thin surface-shell to satura- 

 tion in the opposite direction. The second half-oscillation 

 wipes out some of this opposing magnetism, but to no appre- 

 ciable depth, since the amplitude of the oscillation is by that 

 time greatly reduced. The third half -oscillation tends to 

 magnetize the iron again, and so on. 



When another discharge is passed through the solenoid 

 the first half-oscillation has first of all to demagnetize and 

 magnetize the surface-layers in opposite direction to mag- 

 netism of needle. When it has penetrated through the thin 

 surface-shell the magnetic force meets with a layer of iron of 



