131 
(which should be terminated by cups of mercury) are connected by 
a copper wire not more than a foot in length, no spark is perceived 
when the connection is either formed or broken; but if a wire 
thirty or forty feet long be used (instead of the short wire) though 
no spark will be perceptible when the connection is made, yet when 
it is broken by drawing one end of the wire from its cup of mer- 
cury, a vivid spark is produced.’’ ‘‘ The effect appears somewhat 
increased by coiling the wire into a helix ; it seems also to depend 
in some measure on the length and thickness of the wire. I can 
account for these phenomena only by supposing the long wire to 
become charged with electricity which by its reaction on itself pro- 
jects a spark when the connection is broken.’’* 
In 1875, at the meeting of the National Academy in Philadel- 
phia, I showed to Professor Henry the large electromagnet then re- 
cently acquired by the University of Pennsylvania. I asked him 
to place one of his hands upon one of the magnet-terminals, and 
then, holding the conductor in the other, to break the circuit. He 
did so, and, naturally, received a decided shock. He looked at me 
rather reproachfully, as I thought, for the advantage I had taken of 
him, ‘‘ Pardon me, Professor Henry,’’ I said, ‘‘ but I desired to 
intreduce to you one of your own children. He was a little fellow 
when you knew him and was quite unable to assert himself. But 
now he has grown to man’s estate and is capable, as you see, of deal- 
ing a pretty vigorous blow.’’ With a genial smile, he granted me 
complete absolution. 
One of the most important of Henry’s investigations, made after 
he removed to Princeton, was his research on successive induction, 
an account of which was published in the Proceedings of the Amer. 
Philos. Soc. for November, 1838. In this research he employed 
five annular spools of different sizes of fine wire (about one-fiftieth 
of an inch thick) varying from one-fifth of a mile to nearly a 
mile in length (which might be called intensity helices); and six 
flat spiral coils of copper ribbon, varying from three-quarters of an 
inch to one inch and a half in width and from sixty to ninety-three 
feet in length (which might be called ‘‘ quantity’’ coils). ‘* With 
the single larger ribbon coil in connection with the battery, and 
another ribbon coil placed over it, resting on an interposed glass 
plate, at every interruption of the primary circuit an induction 
spark was obtained at the rubbed ends of the second coil, though 
* Amer. Jour. Science and Arts, xxii, 408, July, 1832. 
