The form usually given to an electro-magnet depends upon the use to which it is to be put. The horseshoe is the most common. This consists of two electro-magnets mounted on a yoke and connected so that the two free poles are North and South. [Illustration: Fig. 85.—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.] Electro-magnets are made on a huge scale for lifting large castings and heavy pieces of iron. Such magnets are used in the great steel mills and in factories where nails, bolts, etc., are manufactured. Monster electro-magnets can be seen in wonderful perfection at the great steel mill at Gary, Indiana. Ships bring the ore down the lakes to Gary, where great steel jaws lift it out of the hold of the boat and carry it to the furnaces. After being melted, great machines pour it out. It is divided into huge ingots, and these, while hot, are carried to the first part of the rolling mill. The ingot is squeezed by a machine, made longer and narrower, then squeezed again and made still longer and narrower. It is started on its journey along the rollers of the mill, squeezed and pressed here and there, as it travels hundreds of yards—no hand ever touching it. It finally arrives, a red-hot steel rail, the right shape and the right length. During this time the steel has made a long journey and changed from a shapeless ingot to a finished rail, handled entirely by machinery guided and controlled by one or two operators, pressing levers and switches. When almost finished, the rail slides down an incline before a man who grasps the rail with huge pinchers, and standing at one end, runs his eye along it. As he looks along the rail he sees the defects, moves the left or the right hand, and another man at the levers of the straightening machine, straightens the rail as directed. And soon there are ten rails, perfectly straight, side by side, with more coming down the incline to meet the glance of the man’s eye. They are still too hot for any man’s touch and so a man sitting in a tower touches an electric switch, and along the overhead rails there comes gliding a monster electro-magnet. The magnet moves along, drops down upon the ten rails, lying side by side and weighing thousands of pounds. The man in the tower presses another switch, thus turning on the current, and electricity glues the rails to the magnet. [Illustration: _By permission, from "Solenoids" by C. R. Underhill._ Lifting-Magnets of the type known as Plate, Billet, and Ingot Magnets.] The ten rails are lifted at once, as easily as you would lift a needle with your horseshoe magnet; they are carried to a flat-car, and when lowered in position, the current is turned off, releasing the rails, and the magnet travels back for another load. Induction