12 CONTRIBUTIONS TO ELECTRICITY AND MAGNETISM. 



doors which separate the laboratory from the lecture room. On the other side 

 of the doors, in the lecture room, and directly opposite the coil, was placed a 

 helix, formed of upwards of a mile of copper wire, one sixteenth of an inch in 

 thickness, and wound into a hoop of four feet in diameter. With this arrange- 

 ment, and a battery of one hundred and forty-seven square feet of zinc surface 

 divided into eight elements, shocks were perceptible in the tongue, when the 

 two conductors were separated, to the distance of nearly seven feet; at the* 

 distance of between three and four feet, the shocks were quite severe. The 

 exhibition was rendered more interesting by causing the induction to take place 

 through a number of persons standing in a row between the two conductors. 



Section II. 

 On apparently two Jcinds of Electro-dynamic Induction. 



32. The investigations arranged under this head had their origin in the 

 following circumstances. After the publication of my last paper, I received, 

 through the kindness of Dr. Faraday, a copy of the fourteenth series of his 

 researches, and in this I was surprised to find a statement which appeared in 

 direct opposition to one of the principal facts of my communication. In para- 

 graph 59, I 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 neutral- 

 ized. 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 non-magnetic m etals. ' ' 



33. As the investigation of the fact mentioned above forms an important part 

 of my paper, and is intimately connected with almost all the phenomena sub- 

 sequently described in the communication, I was, of course, anxious to discover 

 the cause of remarkable a discrepancy. There could be no doubt of the 

 truth of my results, since a shock from a secondary current which would para- 

 lyze 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 



