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 obtained 

 with undiminished intensity. 



Fig. 6. Fig. 7. 



^.«,£1^ 



a a lead plate, of which a a lead plate, h the magnetizing 



the sector h is cut out. spiral. 



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 cur- 

 rent was drawn out, and its direction obtained by the polarity of 

 a needle placed in the spiral at h. 



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

 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 men- 

 tioned, 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 questiori occurs. 

 How does it happen that two currents, both in the same direc- 



