318 CONTRIBUTIONS TO ELECTRICITY AND MAGNETISM. 



was then cut out in the direction of a radius of the circle, as is shown 



Fig- 6. in Fig. 6. With the plate in this condition, no screening 



^ a ^ was produced; the shocks were as intense as if the n)etal 



-b""'^ were not present. 

 kir'^^fatef of 63. This experiment however is not entirely satisfactory, 

 Si'i's?utout since the action might have taken place through the open- 

 ing of the lead ; to obviate this objection, another plate was cut in the 

 same manner, and the two interposed with a glass plate between them, 

 and so arranged that the opening in the one might be covered by the 

 continuous part of the other. Still shocks were obtained with undi- 

 minished intensity. 



64. But the existence of a current in the interposed conductor was 

 rendered certain by attaching the magnetizing spiral by means of two 

 wires to the edge of the opening in the circular plate, as is shown iri 



Fig. 7. Fig. 7. By this arrangement the latent cur- 

 rent was drawn out, and its direction ob- 

 tained by the polarity of a needle placed in 



a represents a lead plate, t the mag- -it 



netizing spiral. thC Spiral at O. 



65. This current was a secondary one, and its direction, in confor- 

 mity with the discovery of Dr Faraday, was found to be the same as 

 that of the primary current. 



66. That the screening influence is in some way produced by the 

 neutralizing action of the current thus obtained, will be clear, from 

 the following experiment. The plate of zinc before mentioned, which 

 is nearly twice the diameter of the helix, instead of being placed be- 

 tween the conductors, was put on the top of the helix, and in this po- 

 sition, although the neutralization was not as perfect as before, yet a 

 great reduction was observed in the intensity of the shock. 



^ 67. But here a very interesting and puzzling question occurs. 

 How does it happen that two currents, both in the same direction, can 

 neutralize each other? I was at first disposed to consider the pheno- 

 menon as a case of real electrical interference, in which the impulses 

 succeed each other by some regular interval. But if this were true 

 the effect should depend on the length and other conditions of the cur- 

 rent in the interposed conductor. In order to investigate this, several 

 modifications of the experiments were instituted. 



