478 Electrostatic Field produced hy Magnetic Induction. 



which I depend upon as a proof of the correctness of the 

 observation. 



Added later {May 10). 



More recently Davies has noticed a peculiar disturbance 

 when working on open circuit, and when the residual elec- 

 trostatic effect is very small ; viz., that it is not constant in 

 direction. It begins with a kick one way, and then a larger 

 motion the other way. By working the reversing-key quickly 

 one kind of swing can be got up; by working it slowly, the 

 other kind of swing can be got up. 



I do not know the meaning of this effect. It is difficult to 

 believe that the fact that the wire is coiled round iron causes 

 the electrostatic charging of its ends to be slow ; though that 

 is the kind of thing it looks like. More probably it is due to 

 some trivial disturbing cause, but the effect is singularly clear 

 and persistent. 



To get over the electrostatic effect more completely, and 

 still without the use of a screen, a single layer of copper ribbon 

 was wound on, in about 10 turns, and its outer turn was put 

 to earth. 



Another disturbing effect was now noticed : it reversed 

 with the current, and it reversed also when the needle was 

 turned through 180°, but it did not reverse when the charge 

 was simply reversed ; nor did it cease when the needle was 

 uncharged. Moreover it was not altogether temporary ; 

 there was a residue permanent. Plainly it was a direct 

 magnetic effect. 



On taking a map of the field with iron filings, a considerable 

 number of lines of force were found to be leaking across the 

 ring, and these would be able to deflect the needle if it had a 

 trace of permanent magnetization ; which it is quite likely to 

 acquire. 



It was taken out and tested betw^een the poles of a powerful 

 magnet — the gelatine w^as distinctly magnetic, the shellac 

 was feebly diamagnetic. 



A number of substances were now tried in order to find 

 something sensibly devoid of magnetic properties, and ulti- 

 mately a paper was found which was barely diamagnetic. 

 With this a new needle was constructed. Wire was once 

 more wound all over the ring, and its field tested with iron 

 filings till hardly any stray lines could be found. 



The mirror was removed, and a small pointer supplied in- 

 stead, so as to be read with a microscope. And in this way 

 the last observations have been made. 



