In all cases of design there is one leading principle which will be
found of great assistance, namely, that a magnet always tends so to
act as though it tried to diminish the length of its magnetic circuit.
It tries to grow more compact. This is the reverse of that which holds
good with an electric current. The electric circuit always tries to
enlarge itself, so as to inclose as much space as possible, but the
magnetic circuit always tries to make itself as compact as possible.
Armatures are drawn in as near as can be, to close up the magnetic
circuit. Many two-pole electromagnets show a tendency to bend together
when the current is turned on. One form in particular, which was
devised by Ruhmkorff for the purpose of repeating Faraday's celebrated
experiment on the magnetic rotation of polarized light, is liable to
this defect. Indeed, this form of electromagnet is often designed very
badly, the yoke being too thin, both mechanically and magnetically,
for the purpose which it has to fulfill.
Here is a small electric bell, constructed by Wagener, of Wiesbaden,
the construction of which illustrates this principle. The
electromagnet, a horseshoe, lies horizontally; its poles are provided
with protruding curved pins of brass. Through the armature are drilled
two holes, so that it can be hung upon the two brass pins; and when so
hung up it touches the ends of the iron cores just at one edge, being
held from more perfect contact by a spring. There is no complete gap,
therefore, in the magnetic circuit. When the current comes and applies
a magnetizing power, it finds the magnetic circuit already complete in
the sense that there are no absolute gaps. But the circuit can be
bettered by tilting the armature to bring it flat against the polar
ends, that being indeed the mode of motion. This is a most reliable
and sensitive pattern of bell.
[Illustration: FIG. 53.--ELECTROMAGNETIC POP-GUN.]
_Electromagnetic Pop-gun._--Here is another curious illustration of
the tendency to complete the magnetic circuit. Here is a tubular
electromagnet (Fig. 53), consisting of a small bobbin, the core of
which is an iron tube about two inches long. There is nothing very
unusual about it; it will stick on, as you see, to pieces of iron when
the current is turned on. 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.