is is intended to represent a
simple bar of iron with a winding of one direction throughout its
length. The gap in the middle of the bar, which divides the winding
into two parts, is intended merely to mark the fact that the winding
need not cover the whole length of the bar and still will be able to
magnetize the bar when the current passes through it. In Fig. 92 a
similar bar is shown with similar winding upon it, but bent into
=U=-form, exactly as if it had been grasped in the hand and bent
without further change. The magnetic polarity of the two ends of the
bar remain the same as before for the same direction of current, and
it is obvious that the portion of the magnetic circuit which extends
through air has been very greatly shortened by the bending. As a
result, the magnetic reluctance of the circuit has been greatly
decreased and the strength of the magnet correspondingly increased.

[Illustration: Fig. 91. Bar Electromagnet]

[Illustration: Fig. 92. Horseshoe Electromagnet]

[Illustration: Fig. 93. Horseshoe Electromagnet]

If the armature of the electromagnet shown in Fig. 92 is long enough
to extend entirely across the air gap from the south to the north
pole, then the air gap in the magnetic circuit is still further
shortened, and is now represented only by the small gap between the
ends of the armature and the ends of the core. Such a magnet, with an
armature closely approaching the poles, is called a _closed-circuit
magnet_, since the only gap in the iron of the magnetic circuit is
that across which the magnet pulls in attracting its armature.

In Fig. 93 is shown the electrical and magnetic counterpart of Fig.
92. The fact that the magnetic circuit is not a single iron bar but is
made up of two cores and one backpiece rigidly secured together, has
no bearing upon the principle, but only shows that a modification of
construction is possible. In the construction of Fig. 93 the armature
_1_ is shown as being pulled directly against the two cores _2_ and
_3_, these two cores being joined by a yoke _4_, which, like the
armature and the core, is of magnetic material. The path of the lines
of force is indicated by dotted lines. This is a very important form
of electromagnet and is largely used in telephony.

_Iron-Clad Form_. Another way of forming a closed-circuit magnet that
is widely used in telephony is to enclose the helix or winding in a
shell of magnetic material which joins the core at one end. This