will be magnetized as in an ordinary magnet.  Likewise, the core will be energized if a current be sent from 5 to 3.  Assuming that the two windings are of equal resistance and number of turns, the effects so produced, when either the coil 1 or the coil 2 is energized, will be equal.  If the battery be connected between the terminals 4 and 5 with the positive pole, say, at 5, then the current will proceed through the winding 2 and tend to generate magnetism in the core in the direction of the arrow.  After traversing the winding 2, however, it will then begin to traverse the other winding 1 and will pass around the core in the opposite direction throughout the length of that winding.  This will tend to set up magnetism in the core in the opposite direction to that indicated by the arrow.  Since the two currents are equal and also the number of turns in each winding, it is obvious that the two magnetizing influences will be exactly equal and opposite and no magnetic effect will be produced.  Such a winding, as is shown in Fig. 96, where the two wires are laid on side by side, is called a parallel differential winding.

Another way of winding magnets differentially is to put one winding on one end of the core and the other winding on the other end of the core and connect these so as to cause the currents through them to flow around the core in opposite directions.  Such a construction is shown in Fig. 97 and is called a tandem differential winding.  The tandem arrangement, while often good enough for practical purposes, cannot result in the complete neutralization of magnetic effect.  This is true because of the leakage of some of the lines of force from intermediate points in the length of the core through the air, resulting in some of the lines passing through more of the turns of one coil than of the other.  Complete neutralization can only be attained by first twisting the two wires together with a uniform lay and then winding them simultaneously on the core.

[Illustration:  Fig. 97.  Tandem Differential Electromagnet]

Mechanical Details.  We will now consider the actual mechanical construction of the electromagnet.  This is a very important feature of telephone work, because, not only must the proper electrical and magnetic effects be produced, but also the whole structure of the magnet must be such that it will not easily get out of order and not be affected by moisture, heat, careless handling, or other adverse conditions.

The most usual form of magnet construction employed in telephony is shown in Fig. 98.  On the core, which is of soft Norway iron, usually cylindrical in form, are forced two washers of either fiber or hard rubber.  Fiber is ordinarily to be preferred because it is tougher and less liable to breakage.  Around the core, between the two heads, are then wrapped several layers of paper or specially prepared cloth