It clearly is an ordinary electromagnet in
that respect. Now suppose I take a little round rod of iron, about an
inch long, and put it into the end of the tube, what will happen when
I turn on my current? In this apparatus as it stands, the magnetic
circuit consists of a short length of iron, and then all the rest is
air. The magnetic circuit will try to complete itself, not by
shortening the iron, but by _lengthening_ it; by pushing the piece of
iron out so as to afford more surface for leakage. That is exactly
what happens; for, as you see, when I turn on the current, the little
piece of iron shoots out and drops down. You see that little piece of
iron shoot out with considerable force. It becomes a sort of magnetic
popgun. This is an experiment which has been twice discovered. I found
it first described by Count Du Moncel, in the pages of _La Lumiere
Electrique_, under the name of the "pistolet electromagnetique;" and
Mr. Shelford Bidwell invented it independently. I am indebted to him
for the use of this apparatus. He gave an account of it to the
Physical Society, in 1885, but the reporter missed it, I suppose, as
there is no record in the society's proceedings.
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