[Illustration:  FIG. 210.—­A wire carrying current attracts iron filings.]

[Illustration:  FIG. 211.—­A loosely wound coil of wire.]

Although such a straight current bearing wire attracts iron filings, its power of attraction is very small; but its magnetic strength can be increased by coiling as in Figure 211.  Such an arrangement of wire is known as a helix or solenoid, and is capable of lifting or pulling larger and more numerous filings and even good-sized pieces of iron, such as tacks.  Filings do not adhere to the sides of the helix, but they cling in clusters to the ends of the coil.  This shows that the ends of the helix have magnetic power but not the sides.

If a soft iron nail (Fig. 212) or its equivalent is slipped within the coil, the lifting and attractive power of the coil is increased, and comparatively heavy weights can be lifted.

[Illustration:  FIG. 212.—­Coil and soft iron rod.]

A coil of wire traversed by an electric current and containing a core of soft iron has the power of attracting and moving heavy iron objects; that is, it acts like a magnet.  Such an arrangement is called an electromagnet.  As soon as the current ceases to flow, the electromagnet loses its magnetic power and becomes merely iron and wire without magnetic attraction.

If many cells are used, the strength of the electromagnet is increased, and if the coil is wound closely, as in Figure 213, instead of loosely, as in Figure 211, the magnetic strength is still further increased.  The strength of any electromagnet depends upon the number of coils wound on the iron core and upon the strength of the current which is sent through the coils.

[Illustration:  FIG. 213.—­An electromagnet.]

[Illustration:  FIG. 214.—­A horseshoe electromagnet is powerful enough to support heavy weights.]

To increase the strength of the electromagnet still further, the so-called horseshoe shape is used (Fig. 214).  In such an arrangement there is practically the strength of two separate electromagnets.

297.  The Electric Bell.  The ringing of the electric bell is due to the attractive power of an electromagnet.  By the pushing of a button (Fig. 215) connection is made with a battery, and current flows through the wire wound on the iron spools, and further to the screw P which presses against the soft iron strip or armature S; and from S the current flows back to the battery.  As soon as the current flows, the coils become magnetic and attract the soft iron armature, drawing it forward and causing the clapper to strike the bell.  In this position, S no longer touches the screw P, and hence there is no complete path for the electricity, and the current ceases.  But the attractive, magnetic power of the coils stops as soon as the current ceases; hence there is nothing to hold the armature down, and it flies back to its former position.  In doing this, however, the