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
construction results in what is known as the _tubular_ or _iron-clad_
electromagnet, which is shown in section and in end view in Fig. 94.
In this the core _1_ is a straight bar of iron and it lies centrally
within a cylindrical shell _2_, also of iron. The bar is usually held
in place within the shell by a screw, as shown. The lines of force set
up in the core by the current flowing through the coil, pass to the
center of the bottom of the iron shell and thence return through the
metal of the shell, through the air gap between the edges of the shell
and the armature, and then concentrate at the center of the armature
and pass back to the end of the core. This is a highly efficient form
of closed-circuit magnet, since the magnetic circuit is of low
reluctance.

[Illustration: Fig. 94. Iron-Clad Electromagnet]