208 Prof. J. Henry's Coiitrihutions 



netic. The wire of this spool is Jg^th of an inch thick, and 

 we therefore see from this experime-nt, 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. 1, 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. I, 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. 1, 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- 



