[Illustration: Fig. 79.—Magnetic Phantom formed about a Wire carrying a Current of Electricity.] This is readily shown by punching a small hole in a piece of cardboard, and passing a wire carrying a strong current of electricity through the hole. If a few iron filings are sifted on the cardboard and the latter jarred slightly with a pencil as they fall, they will arrange themselves in circles with the wire at the center, forming a magnetic phantom and showing the paths of the lines of magnetic force. [Illustration: Fig. 80.—Magnetic Phantom formed about several Turns of wire.] By forming the wire into a coil as in Figure 80 the magnetic field generated is much stronger and more plainly seen, for then the combined effect of the wires is secured. [Illustration: Fig. 81.—Paper Tube wrapped with Wire for Experimental Purposes.] Roll up a small paper tube about 1/2 inch in diameter and four inches long. Wind neatly on the tube three layers of No. 18 insulated copper wire. Pass an electric current through it from two or three cells of a battery, and test its magnetic properties by bringing it near a compass needle. It will be found that the coil possesses very marked magnetic properties, and will readily cause the needle to swing about, even though it is held quite a distance away. If an iron bar is placed inside of the paper tube, the magnetic effect will be greatly increased. [Illustration: Fig. 82.—Showing how the Lines of Force "Leak" at the sides of the coil, from a Coil of Wire, and how they are concentrated by an Iron Core.] The presence of the iron bar inside of the coil of wire greatly increases the number of lines of force running through the coil. [Illustration: Fig. 83.—The Principle of an Electro-Magnet.] When a bar is not used, many of the lines of force leak out at the sides of the coil, and but few extend from end to end. The effect of the iron core is not only to diminish the leakage of the lines of force, but also to add many more to those previously existing. Hence the magnetic strength of a coil is greatly increased by the iron core. A coil of wire wrapped around an iron core forms an _electro-magnet_. [Illustration: Fig. 84.—if you wrap some insulated Wire around an Ordinary Nail and connect it to a Battery, it will become an Electro-Magnet.] If you wrap some insulated wire around an ordinary nail and connect it to one or two cells of a battery it will become an electro-magnet and pick up bits of iron and steel. If you wind the wire around a small paper tube into which a nail will slide easily, the coil will draw the nail in when the current is turned On. A hollow coil of this sort is called a solenoid. Electro-magnets and solenoids play a part in the construction of almost all electrical machinery. They form the essential parts of dynamos, motors, telephone receivers, telegraph relays and sounders, and a host of other devices.