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 ;1,th 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, 
heither 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 ;,th 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 
_ Iny 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. xxxvint, No. 2.—Jan.—March, 1840. 28 
