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Prof. J. Henry’s Contributions 
netic. The wire of this spool is J^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 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 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 increased, the power of giving shocks may also 
become neutralized. 
36. The inductive action of coil No. J, in the foregoing ex- 
periments, is precisely the same as that of a temporary mag- 
net 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 in- 
vestigated by Sturgeon and Lenz. 
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 quantity and feeble intensity ; ,but 
succeeding experiments will show that this is not necessarily 
the case. 
38. All the experiments given in this section have thus far 
been made with a battery of a single element. This condi- 
tion was now changed, and a Cruickshanks trough of sixty 
pairs substituted. When the current from this was passed 
through the riband coil No. 1, no indication, or a very feeble 
one, was given of a secondary current in any of the coils 
or .helices, arranged as in the preceding experiments. The 
length of the coil, in this case, was not commensurate with 
the intensity of the current from the battery. But when the 
long helix. No. 1, was placed instead of coil No. 1, a power- 
ful inductive action was produced on each of the articles, as 
before. 
39. First, helices No. 2 and 3 were united into one, and 
placed within helix No. i, which still conducted the battery 
current. With this disposition a secondary current was pro- 
duced, which gave intense shocks but feeble decomposition, 
and no magnetism in the soft iron horseshoe. It was there- 
