ON ELECTRO-DYNAMIC INDUCTION. 311 



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 perceptible, 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 

 magnetic. The wire of this spool is Tsth 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 increased, with an increased 

 effect. 



35. The fact (33) that the induced current is diminished by a fur- 

 ther increase of the wire, after a certain length has been attained, is 

 important in the construction of the magneto-electrical machine, since 

 the same effect is produced in the induction of magnetism. Dr God- 

 dard of Philadelphia, to whom I am indebted for coil No. 5, found 

 that when its whole length was wound on the iron of a temporary 

 magnet, no shocks could be obtained. The wire of the machine may 

 therefore be of such a length, relative to its diameter, as to produce 

 shocks, but no decomposition ; and if the length be still further in- 

 creased, the power of giving shocks may also become neutralized. 



36. The inductive action of coil No. 1, in the foregoing experiments, 

 is precisely the same as that of a temporary magnet in the case of the 

 magneto-electrical machine. A short thick wire around the armature 

 gives brilliant deflagrations, but a long one produces shocks. This fact, 

 I believe, was first discovered by my friend Mr Saxton, and afterwards 

 investigated by Sturgeon and Lentz. 



37. We might, at first sight, conclude, from the perfect similarity 

 of these effects, that the currents which, according to the theory of 

 Ampere, exist in the magnet, are like those in the short coil, of great 



VI. — 4 c 



