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!" [Illustration: 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. "By means of an instrument which works like the motorman's controller on the electric car, I may control the amount of electricity which flows, just as well as you may control the flow of water by a faucet or stop-cock. By this means I will control the strength of the magnet so that you may move the armature in your dynamo experiment. "In 1821, Faraday, at the Royal Institution, London, learned that he could produce magnetism by means of the electric current, and, in 1831, he learned that the reverse was also true, namely, that he could produce electricity from magnetism. This idea coming as the result of ten years of incessant search made him shout and dance like a child. You are feeling a little of the pleasure of his discovery." [Illustration: Fig. 4] I then fastened one of the coils upon the table underneath a small bench (Fig. 4) and sent an electric current around it. The other coil, _B_, connected with the ammeter was pushed back and forth along the surface of the bench over this coil. The boys found that the more electric current I sent around the coil _A_, that is, the stronger I made the magnetic field, the harder it was to move the coil _B_. They found that the nearer _B_ was to _A_ the harder it was to move it. They found that the faster they moved _B_ the more electricity was produced. They tried laying _B_ upon its side upon the bench and thus moving it. They tried taking _B_ off the bench and moving it on all sides of _A_. They found it much harder to move in some ways than in others, but in all cases they found that the harder they had to work the more electricity was developed, as was shown by the ammeter. "The dynamo is any machine which will convert mechanical work into electricity. The magneto is one form of a dynamo which you have used much at the summer cottage, but have never seen the inside of. Here are several (see Figs. 5, 6, and 8) which I will let you examine inside and out, and with these I must leave you to yourselves for a time."