"But thanks to Oersted in Denmark in 1819 and Faraday in England in 1821 and Joseph Henry in Albany, N. Y., in 1827, we have learned to make exceedingly powerful magnets by sending a current of electricity in a whirl around the iron. This is the meaning of the coils of copper wire around iron cores in the dynamo, in electric bells, in telegraph sounders, in motors, etc., etc. To prevent the electric current from taking the shortest route, through the iron core or through the successive layers of copper wire, the iron core and the wire must be covered with something like wood or paper or cotton or silk or rubber--such things as electricity does not readily pass through--that is, insulating material. "Joseph Henry, while teaching in the Albany Academy, was the first to make electro-magnets. There was no such thing as wire covered with an insulating material then in the market, and he wound all his wire with silk ribbon. But in the year 1834 he made magnets which lifted thirty-five hundred pounds, to the astonishment of every one. A pair of such electro-magnets as I have here (Fig. 3), each consisting of one pound of No. 24 cotton covered copper wire, eight hundred feet long, wound in one thousand turns about an iron core two inches in diameter, will lift several hundred pounds: much more than we three can lift, as I shall now show you." [Illustration: Fig. 3] 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."