490 Transactions. — Miscellaneous. 



low permeability, since the greater part is already magnetized 

 nearly to saturation in the same direction by the action of the 

 first half-oscillation of the first discharge. It therefore pene- 

 trates further, for we know the magnetic force penetrates 

 deeper in a conductor like copper (/x = 1) than in a conductor 

 like iron, where //. may be considerable. More iron is demag- 

 netized and the deflection reduced. This continues as spark 

 after spark is passed, till finally the discharge cannot pene- 

 trate any further. This corresponds to the steady state. It 

 was found, by dissolving a needle acted on in this way by a 

 succession of discharges, that the deflection rose steadily as 

 the needle was eaten away, showing that the surface-layer 

 was magnetized in an opposite direction to the central part. 



In the experiment above detailed it was found that the 

 discharge had penetrated to about one-quarter of the radius — 

 i.e., a distance of O008in. When thin steel needles were 

 experimented on they were often totally demagnetized and 

 magnetized in the opposite direction by successive dis- 

 charges : e.g., thin steel needle, 0008m. diameter : — 



(1.) Effect of second half-oscillation and succeeding — 



Sparks. Deflection. 



... ... ... ... 150 



1 ... ... .. ... 80 



2 ... ... ... ... 55 



3 ... ... ... 50 &c. 



(2.) Effect of first half-oscillation and succeeding — 



Sparks. Deflection. 



... ... ... ... 150 



1 -15 



2 ... . ... ... -120 



3 ... ... ... ... -140 



Soft iron as well as steel needles exhibited the same 

 effect. 



The difference between the effect of the first and the second 

 half-oscillation in demagnetizing iron is very marked. The 

 experiments show clearly how rapidly the oscillations decay 

 in amplitude. When we are dealing with capacities of about 

 1,000 electrostatic units and small inductance in the circuit 

 it seems very probable that there is only one complete oscilla- 

 tion. The others are damped down to such an extent as to be 

 inappreciable. The fact that the deflection due to the needle 

 always falls, whatever the direction of the first oscillation, 

 shows clearly that the discharge is oscillatory. If there was 

 only a unidirectional discharge the needle should only be 

 affected when the discharge is in one direction. 



Simple experiments of this nature on ordinary steel needles 



