pended by a string passing over a pulley, and then gradually lowered down towards 

 the plane of the coil, until the shocks are of the required intensity." ^^ 



b. Action of Induction at a Distance 



Henry investigated the action of the discharge of a Leyden jar 

 through a primary circuit in inducing currents in distant secondary 

 circuits. For example, the following report of a paper read in 1842 

 was published the following year: 



"... a single spark . . . thrown on the end of a circuit of wire in an upper room, 

 produced an induction sufficiently powerful to magnetize needles in a parallel circuit 

 of wire placed in the cellar beneath, at a perpendicular distance of thirty feet with two 

 floors and ceilings . . . intervening when it is considered that the mag- 

 netism of the needle is the result of the difference of two actions, it may be further 

 inferred that the diffusion of motion in this case is almost comparable with that of a 

 spark from a flint and steel in the case of light." '' 



These experiments were carried to greater distances as is illustrated 

 by the following excerpt from the proceedings of the meeting of the 

 American Philosophical Society, October 21, 1842: 



"Prof. Henry communicated, orally, an extension of the experiments, which 

 he had previously brought before the Society, on electro-dynamic induction. He had 

 succeeded in magnetizing needles by the secondary current in a wire more than two 

 hundred and twenty feet distant from the wire through which the primary current 

 was passing, excited by a single spark from an electrical machine." *" 



Also, in the notes on "Lectures on Natural Philosophy by Professor 

 Henry" made by William J. Gibson, a student, February 28, 1844, 

 after a description of experiments on induction at a distance in which 

 inductive effects were noted several hundred feet away, it is stated: 



"Hence the conclusion that every spark of electricity in motion e.xerts these in- 

 ductive effects at distances indefinitely great (effects apparent at distances of one-half 

 a mile or more); and another ground for the supposition that electricity pervades all 

 space. Each spark sent off from the Electrical Machine in the College Hall sensibly 

 affects the surrounding electricity through the whole village. A fact no more improb- 

 able than that light from a candle (probably merely another kind of wave or vibration 

 of the same medium), should produce a sensible effect on the eye at the same dis- 

 tance." *^ 



This is also referred to in Henry's letter to Rev. S. B. Dod, written 

 in 1876 as follows: 



"As another illustration of this, it may be mentioned that when a discharge of a 

 battery of several Leyden jars was sent through the wire before mentioned, stretched 

 across the campus in front of Nassau Hall, an inductive effect was produced in a 

 parallel wire, the ends of which terminated in the plates of metal in the ground in the 

 back campus, at a distance of several hundred feet from the primary current, the 

 building of Nassau Hall intervening. The effect produced consisted in the magnet- 

 ization of steel needles." ^ 



38 SW, Vol. 1, p. 121. 



^i-SW, Vol. 1, p. 203. 



^'^ Proceedings of the American Philosophical Society, Vol. 2, Jan., 1841-May, 184.S. 



^^ Manuscript in Princeton Library, p. 135. 



^ A Memorial of Joseph Henry, published by order of Congress, 1880, pp. 151-152. 



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