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