258 
Prof. J, Henry’s Contributions 
it became more intense. Thus a sheet of tinfoil interposed 
produced no perceptible influence ; 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 
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 ^ ^ ,,,3 
lead ; to obviate this oojection, another plate pi^te, 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 5. 
65. This current was a se- 
condary one, and its direction,- 
in conformity with the dis- « represents a lead plate, 6 the raag- 
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 
