On Electro- Dynamic Induction. 217 



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

 ameter. Helix No. 5, which consists of wire only t^s^h of an 

 inch in diameter, was placed on coil No. 2, and its length in- 

 creased 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, (10,) 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 y'gth 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 

 further increase of the wire, after a certain length has been at- 

 tained, is important in the construction of the magneto-electrical 

 machine, since the same effect is produced in the induction of 

 magnetism. Dr. Goddard of Philadelphia, to whom I am in- 

 debted 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 de- 

 composition ; and if the length be still further increased, the 

 power of giving shocks may also become neutralized. 



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

 iments, 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 simi- 

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

 theory of Ampere, exist in the magnet, are like those in the short 



Vol. XXXVIII, No. 2.— Jan.-March, 1S40. 28 



