Electromagnet: Horseshoe Holding Magnet Sized by Its Pull (Underhill, 1903)

Pull P (pounds) = B² × A / 72,134,000, where B is the flux density in lines per square inch and A the pole area in square inches. Working densities: wrought iron 90,000, cast steel 85,000, Mitis iron 80,000, ordinary cast iron 50,000, which give 112.3, 100.1, 88.72 and 34.66 lb per square inch of pole. Divide the pull you need by that figure to get the pole area. Underhill's example, for cast steel taken at 90,000: 10 lb needs 0.089 square inch, a core 0.336 inch across. A two-pole magnet holds twice the weight of a single pole at the same induction.

Make the practical horseshoe magnet in three pieces besides the armature: two round cores and a yoke (back iron) to join them. Make an armature to lie across the two pole ends. Face the poles and the armature accurately; an uneven contact spreads the lines unevenly and weakens the pull. Ours: wrought iron. Not given: sizes, how the cores are fastened.

Design the coil to stand the full supply voltage without overheating; never put a resistor in series with the magnet, as that wastes power and makes the magnet weaker. Pick the wire size that makes the coil's resistance equal to that of the line and the source, then try the next larger size, and keep the one giving more ampere-turns. A coil rises about 100 °F at about 0.9 watt per square inch of surface for a thin telephone-ringer coil, or 0.33 for a thick one (4 1/8 in across, 7 in long, 1 7/16 in thick), and the rise grows in proportion to the watts. Ours: the supply, e.g. 12 V.

Wind the wire directly onto each core between insulating end heads, and fasten the wound cores to the yoke. The joints between cores and yoke lose a little pull. Ours: enamelled copper wire. Not given: the winding method, the insulation.

Connect the coils to the supply and put the armature against the faced poles: the magnet holds it with the pull it was sized for. When the armature touches the poles there is practically no leakage between them.