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]

Such forms of magnets are frequently used where it is necessary to
mount a large number of them closely together and where it is desired
that the current flowing in one magnet shall produce no inductive
effect in the coils of the adjacent magnets. The reason why mutual
induction between adjacent magnets is obviated in the case of the
iron-clad or tubular magnet is that practically all stray field is
eliminated, since the return path for the magnetic lines is so
completely provided for by the presence of the iron shell.

_Special Horseshoe Form._ In Fig. 95 is shown a type of relay commonly
employed in telephone circuits. The purpose of illustrating it in this
chapter is not to discuss relays, but rather to show an adaptation of an
electromagnet wherein low reluctance of the magnetic circuit is secured
by providing a return leg for the magnetic lines developed in the core,
thus forming in effect a horseshoe magnet with a winding on one of its
limbs only. To the end of the core _1_ there is secured an =L=-shaped
piece of soft iron _2_. This extends upwardly and then forwardly
throughout the entire length of the magnet core. An =L=-shaped armature
_3_ rests on the front edge of the piece _2_ so that a slight rocking
motion will be permitted on the "knife-edge" bearing thus afforded. It
is seen from the dotted lines that the magnetic circuit is almost a
closed one. The only gap is that between the lower end of the armature
_3_ and the front end of the core. When the coil is energized, this gap
is closed by the attraction of the armature. As a result, the rearwardly
projecting end of the armature _3_ is raised and