124 WRITINGS OF JOSEPH HENRY. [1838 



62. A circular plate of lead was interposed, which caused 

 the induction in the helix almost entirely to disappear. A 

 slip of the metal was then cut out in the direction of a radius 



^ -,,^ of the circle, as is shown in Fig. 6. With the 



Cv^^^^ plate in this condition, no screening was pro- 

 _ * duced ; the shocks were as intense as if the 



Fia. 6. — a rep- , , 



resents a lead metal were not present. 



plate, of which 33^ This experiment however is not entirely 



the sector is . / . • i 1 



cut out. satisfactory, since the action might have taken 



place through the opening of the lead ; to obviate this ob- 

 jection, another plate was cut in the same manner, and the 

 two interposed with a glass plate between them, apd so ar- 

 ranged that the opening in the one might be covered by the 

 continuous part of the other. Still shocks were obtained 

 with undiminished intensity. 



64. But the existence of a current in the interposed con- 

 ductor 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 in Fig. 7. By this arrange- 

 ment the latent current was drawn 

 out, and its direction obtained by 

 the polarity of a needle placed in 



Fig. 7. — a represents a lead plate, the spiral at h. 



h the magnetizing spiral. Q^ ^j^-g ^.^^rent was a second- 



ary one, and its direction, in conformity 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 between the conductors, was 

 put on the top of the helix, and in this position, 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 oc- 

 curs. How does it happen that two currents, both in the 

 same direction, can neutralize each other? I was at first 



