Another form of the electric magnet is shown in cross-section in [Fig. 3]. This consists of a short steel cylinder with a groove in its face for the electric coil. The modern lifting magnet is a highly specialized form of this type of electric magnet.
Although the use of a magnet for lifting purposes seems to be a very simple idea and easily adopted, many difficulties had to be overcome and years of experimenting done before the lifting magnet was a commercial success. Nearly all electrical machinery may easily be protected from rough usage and moisture, but the lifting magnet must be so strongly designed that it will withstand the countless blows due to heavy pieces of iron flying against it, and the banging it must get against the sides of cars, ships, etc. All light parts must be placed inside of the magnet or in such a position that they can never be knocked off or broken. To moisture in some form or other nearly all lifting-magnet troubles can be traced. Hence the importance of an absolutely moisture-proof construction. The result of moisture in the interior of a magnet is to weaken the effectiveness of the installation, leading eventually to short circuits and burn-outs. It is necessary not only to guard against moisture in the form of rain, snow or dew, but precaution must also be taken against the entrance into the magnet of moisture-laden air, since moisture so introduced will presently be condensed in the form of drops of water.
Fig. 3
A 43-Inch Magnet Handling Pig Iron
A very natural question is, how much such a magnet will lift. For a given size of magnet, the lifting capacity varies greatly with the nature of the load handled. With a magnet sixty-two inches in diameter, this may vary from in the neighborhood of 1,000 pounds for light scrap, to from 4,000 to 5,000 pounds for pig iron, and as high as 60,000 pounds for a solid mass of steel or iron such as, for instance, a skull-cracker ball or a casting affording surface for good magnetic contact.