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]

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