"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."
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."
When I returned I asked the boys why these dynamos were called magnetos. "Because they have steel magnets for their fields," they replied. "There are several magnets bent in the shape of a horseshoe."
"Yes," I said, "in this case the field is made stronger by taking several magnets. Have you noticed any armature?" "Yes, it is made of iron with insulated copper wire wound around it."
"Please recall that the amount of energy you expend in going upstairs depends on two things: (1) your weight and (2) the speed with which you move. Also recall that the amount of electricity you could generate with a dynamo depended upon the amount of energy you expended. Therefore, the strength of the electric current which this machine may produce depends upon two things: (1) the strength of the magnetic field against which you must pull and (2) the speed of the motion of the armature. Evidently this field is made as strong as it is possible to make it with steel magnets. Now is there any device for giving high speed to the armature?"