154 WRITINGS OP JOSEPH HENRY. [1840 



next shorter length it was more perceptible, and increased 

 in intensity with each diminution of the coil, until a length 

 of about fifteen feet appeared to give a maximum result. 



12. The diminution of the intensity of the shock in the 

 last experiment, after the length of the coil was diminished 

 below fifteen feet, was due to the diminution of the number 

 of spires of the coil, each of which, by acting on the helix, 

 tends to'increase the intensity of the secondary current, un- 

 less the combined length of the whole is too great for the in- 

 tensity of the battery. That this is the fact is shown by the 

 following experiment: the helix was placed on a single spire 

 or turn of the coil, and the length of the other part of the 

 copper ribbon, which did not act on the helix, was contin- 

 ually shortened, until the whole of it was excluded from the 

 circuit ; in this case the intensity of the shock at the begin- 

 ning was constantly increased. We may therefore state 

 generally, that at the beginning of the battery current, the 

 induction of a unit of its length is increased by every 

 diminution of the length of the conductor. 



13. In the experiment given in paragraph 11, the inten- 

 sity of the shock at the ending of the battery current dimin- 

 ishes with each diminution of the length of the coil ; and 

 this is also due to the decrease of the number of the spires of 

 the coil, as is evident from an experiment similar to the last, 

 in which the helix was placed on a coil consisting of only 

 two turns or spires of copper ribbon ; the shock at the end- 

 ing, with this arrangement, was comparativel}'- feeble, but 

 could be felt in the hands. Different lengths of coil No. 2 

 were now introduced into the same circuit, but not so as to 

 act on the helix ; but although these were varied from four 

 or five feet to the whole length of the coil, (sixty feet,) not 

 the least difference in the intensity of the shock could be 

 perceived. We have therefore the remarkable result, that 

 the intensity of the ending induction of each unit of length 

 of the battery current is not materially altered, at least within 

 certain limits, by changing the length of the whole con- 

 ductor. From this we would infer that the shock depends 

 more on the intensity of the action than on the quantity of 



