ll 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.

ELECTROMAGNETS FOR USE WITH ALTERNATING CURRENTS.

When you are designing electromagnets for use with alternating
currents, it is necessary to make a change in one respect, namely, you
must so laminate the iron that internal eddy currents shall not occur;
indeed, for all rapid-acting electromagnetic apparatus it is a good
rule that the iron must not be solid. It is not usual with telegraphic
instruments to laminate them by making up the core of bundles of iron
plates or wires, but they are often made with tubular cores, that is
to say, the cylindrical iron core is drilled with a hole down the
middle, and the tube so formed is slit with a saw cut to prevent the
circulation of currents in the substance of the tube. Now when
electromagnets are to be employed with rapidly alternating currents,
such as are used for electric lighting, the frequency of the
alternations being usually about 100 periods per second, slitting the
cores is insufficient to guard against eddy currents; nothing short of
completely laminating the cores is a satisfactory remedy. I have here,
thanks to the Brush Electric Engineering Company, an electromagnet of
the special form that is used in the Brush arc lamp when required for
the purpose of working in an alternating current circuit. It has two
bobbins that are screwed up against the top of an iron box at the head
of the lamp. The iron slab serves as a kind of yoke to carry the
magnetism across the top. There are no fixed cores In the bobbins,
which are entered by the ends of a pair of yoked plungers. Now in the
ordinary Brush lamp f