162 WHITINGS OF JOSEPH HENRY. [1840 



peared in direct opposition to one of the principal facts of 

 my communication. In paragraph 59, 1 state in substance 

 that when a plate of metal is interposed between the coil 

 transmitting a galvanic current, and the helix placed above 

 it to receive the induction, the shock from the secondary- 

 current is almost perfectly neutralized. Dr. Faraday, in the 

 extension of his new and ingenious views of the agency of the 

 intermediate particles in transmitting induction, was led to 

 make an experiment on the same point, and apparently, 

 under the same circumstances, he found that it " makes not 

 the least difference whether the intervening space between 

 the two conductors is occupied by such insulating bodies as 

 air, sulphur, and shell-lac, or such conducting bodies as 

 copper and other nonrmagnetic metals." 



33. As the investigation of the fact mentioned above 

 forms an important part of my paper, and is intimately con- 

 nected with almost all the phenomena subsequently de- 

 scribed in the communication I was of course anxious to 

 discover the cause of so remarkable a discrepancy. There 

 could be no doubt of the truth of my results, since a shock 

 from a secondary current which would paralyze the arms 

 was so much reduced by the interposition of plates of metal 

 as scarcely to be felt through the tongue. 



34. After some reflection however the thought occurred to 

 me that induction might be produced in such a way as not 

 to be affected by the interposition of a plate of metal. To 

 understand this, suppose the end of a magnetic bar placed 

 perpendicularly under the middle of a plate of copper, and 

 a helix suddenly brought down on this; an induced current 

 would be produced in the helix by its motion towards the 

 plate, since the copper, in this case, could not screen the 

 magnetic influence. Now, if we substitute for the magnet 

 a coil through which a galvanic current is passing, the 

 effect should be the same. The experiment was tried by 

 attaching the ends of the helix to a galvanometer,* and the 



•'^■The arrangement will be readily understood by supposing in Fig. 3, the 

 handles removed, and the ends of the helix joined to the ends of the wire of 

 a galvanometer; also, by a plate of metal interposed between the helix and 

 the coil. 



