258 Prof. J. Henry's Contributions 



it became more intense. Thus a sheet of tinfoil interposed 

 produced no perceptible influence j also four sheets of the 

 same were attended with the same result. A certain thick- 

 ness of metal is therefore required to produce the screening 

 effect, and this thickness depends on the quantity of the cur- 

 rent from the battery. 



61. The idea occurred to me that the screening might, in 

 some way, be connected with an instantaneous current in the 

 plate, similar to that in the induction by magnetic rotation, 

 discovered by M. Arago. The ingenious variation of this 

 principle by Messrs. Babbage and Herschel, furnished me 

 with a simple method of determining this point. 



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 plate in this 

 condition, no screening was produced ; the j^. 

 shocks were as intense as if the metal were ^' ' 



not present. 



63. This experiment however is not en- 

 tirely satisfactory, since the action might have 

 taken place through the opening of the ^ ^^ ,^ , i,,^ 

 lead ; to obviate this objection, another plate piate, of which the 

 was cut in the same manner, and the two in- sector b is cut out. 

 terposed 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 ob- 

 tained 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 arrangement 

 the latent current was drawn 



out, and its direction obtained Fig. 7. 



by the polarity of a needle 

 placed in the spiral at b. 



65. This current was a se- 

 condary one, and its direction, 



in conformity with the dis- a repi-esents a lead plate, b the mag- 

 covery of Dr. Faraday, was netizing spiral 



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 



