116 WRITINGS OF JOSEPH HENRY. [1838 



with the apparatus before mentioned, disappeared. The 

 sparks were much smaller, and also the decomposition less, 

 than with the short coil ; but the shock was almost too intense 

 to be received with impunity, except through the fingers of 

 one hand. A circuit of fifty-six of the students of the senior 

 class, received it at once from a single rupture of the battery 

 current, as if from the discharge of a Leyden jar weakly 

 charged. The secondary current in this case was one of 

 small quantit}^, but of great intensity. 



33. The following experiment is important in establishing 

 the fact of a limit to the increase of the intensity of the shock, 

 as well as the power of decomposition, with a wire of a given 

 diameter. Helix No. 5, which consists of wire only yj^^^ 

 of an inch in diameter, was placed on coil No. 2, and its 

 length increased to about seven hundred yards. With this 

 extent of wire, neither decomposition nor magnetism could 

 be obtained, but shocks were given of a peculiarly pungent 

 nature ; they did not however produce much muscular action. 

 The wire of the helix was further increased to about fifteen 

 hundred yards; the shock was now found to be scarcely per- 

 ceptible in the fingers. . 



34. As a counterpart to the last experiment, coil No. 1 was 

 formed into a ring of sufficient internal diameter to admit 

 the great spool of wire (11), and with the whole length of 

 this (which, as has before been stated, is five miles) the shock 

 was found so intense as to be felt at the shoulder, when 

 passed only through the forefinger and thumb. Sparks and 

 decomposition were also produced, and needles rendered mag- 

 netic. The wire of this spool is yVth of an inch thick, and 

 we therefore see from this experiment, that by increasing 

 the diameter of the wire, its length may also be much in- 

 creased, with an increased effect. 



35. The fact (33) that the induced current is diminished 

 by a further increase of the wire, after a certain length has 

 been attained, is important in the construction of the mag- 

 neto-electrical machine, since the same effect is produced in 

 the induction of magnetism. Dr. Goddard of Philadelphia, 

 to whom I am indebted for coil No. 5, found that when its 



