224 Contributions to Electricity and Magnetism. 
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 entirely satisfactory, since 
the action might have taken place through the opening 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 obtain 
with undiminished intensity. 
Fig. 6. 
aa lead site, of which aa lead plate, b the magnetizing 
the sector b is cut out. spiral. eo 
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 in Fig. 7. By this arrangement the latent cul 
rent was drawn out, and its direction obtained by the polarity of 
a needle placed in the spiral at b. 
65. This current was a secondary one, and its direction, 11 COD" 
formity 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 mer 
tioned, which is nearly twice the diameter of the helix, instead 
of being placed between the conductors, was put on the top © 
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 occurs: 
How does it happen that two currents, both in the same direc 
