372 Mr. J. J. Waterston on some Electrical Experiments. 



edges of the B. D. and cap. Next let us take a metallic 

 disk of the same diameter as the cake, and having also applied 

 similar insulating handles, introduce it and move it up and down 

 between the B. D. and cap without touching either. The leaves 

 of the electroscope will be found to be acted upon in the same 

 way as before, but in a greater degree. 



I submit that if the sulphur had a greater capacity than air 

 of transmitting induction, the leaves of the electroscope ought 

 to rise higher when the cake is close to the charged B. D. than 

 when it is close to the cap of the electroscope. In the latter case 

 the increase of the angle of the gold-leaves was fully as great as 

 when the electroscope was brought half an inch nearer to the 

 B. D. In the former case no increase of induction was apparent. 



The same experiment, repeated with sealing-wax and gutta- 

 percha plates, showed similar results, but more striking. When 

 close to B. D. the inductive force was less than through air; 

 when close to the cap it was greater. [Gutta percha shows some 

 curious abnormal effects, especially when heated and soft. 

 Although it is then still a perfect non-conductor, it seems to 

 absorb the inductive force like a sponge.] A change in the 

 shape or size of the non-conducting plates modifies the result, 

 exactly in the same manner as the conducting plates, though 

 always in a less degree. The proper way of studying the sub- 

 ject would therefore seem to be to experiment with conducting 

 and non-conducting plates of the same size, and guard against 

 the effect of induction upon the non-conducting surfaces, which 

 seems to be a phenomenon of a special kind, that stands in need 

 of further elucidation. I have not been able to arrive at any 

 distinct law of action. 



The transition from one kind of induction to the other may 

 be conveniently studied in a warm room during dry weather, by 

 placing a shade of thin glass over the electroscope, of such size 

 that the round top may be about an inch above the cap, the 

 agent being an insulated brass disk (the B. D.) charged induc- 

 tively from an excited plate of sulphur. If we moisten the out- 

 side of the shade with a sponge and bring the excited B. D. close 

 to the top, but not touching, there is of course no effect on the 

 electroscope, the surface is opake to the electric force ; the in- 

 duction is confined to the aqueous surface opposite the B. D. 

 As the shade dries we observe a slight effect on the gold-leaves, 

 which becomes more and more apparent as the glass resumes 

 the dielectric condition. Let the shade be now heated before a 

 clear fire, and replaced, when cool, over the electroscope. We 

 may now study the effect of moving the excited B. D. to and 

 from the top of the shade. If the action is performed without 

 pause, the leaves open and shut in the regular normal way ; but 



