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.
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