The cores of the two magnets were bolted fast to an iron beam, and a large bar of iron with a ring in it was laid across the other free ends of the magnet cores. I made connections with the electric lighting circuit (that in my laboratory is what is called a direct current), and sent a current of electricity around the coils. The two boys and I tugged at the ring in the iron bar to no avail. We were unable to pull the iron bar away from the magnet. But when I opened the switch and cut off the electric current, one boy with one finger in the ring lifted the bar with perfect ease.
"Electro-magnets are now made with a magnetic intensity 90,700 times that of the earth's magnetism. Electro-magnets are used for hoisting iron castings weighing many tons. Here is a picture of an electro-magnet lifting a whole wagon load of kegs of nails from the wagon to the hold of a ship.
"Electro-magnets are our only means of utilizing electricity for power. It is the pull of electro-magnets that moves the electric car. Electro-magnets are now used for pulling all the trains out of the Grand Central Depot in New York City.
"Let us now compare the strength of our electro-magnet with that of the bar magnet used in our former experiment."
I opened and closed the switch, which sent the electric current through my magnet coils at frequent intervals, and the two boys, each with a compass needle, searched the field for magnetic effects. They found that the magnetic field extended six or eight feet, but this piece of research was broken up by a new idea which appeared to strike them both at the same instant, for they shouted both together, "Let's use this electro-magnet in place of the bar magnet for our dynamo experiment!"
Photograph by Helen W. Cooke
Wiring
"That is surely the next step in our programme," said I, "but you will need a steam-engine to move an armature in this magnetic field, will you not, judging from the struggle we had with that iron bar a few minutes ago?" The boys looked quite hopeless until I said, "The best thing about the electro-magnet remains yet to be told. You have perfect control of its strength by changing the amount of electricity which you send around the coil.