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?" "Yes, indeed," said the boys, "one has a pulley so that it may be connected by a belt with a gas engine, and the others have each a large cog-wheel working into a smaller one. We found in one of them that a single revolution of the crank gave six revolutions to the armature." I found that the boys had made large-sized drawings of the parts, and were preparing to report on the magneto as a form of dynamo at the next meeting of the Science Club, which we had started among the boys in school. [Illustration: Fig. 5] "I will loan you some apparatus so that you may give a very interesting demonstration on that subject," said I, "only let me show you how to use it first. Connect the binding posts _D_ and _E_ of this magneto (Fig. 5) with my ammeter. Turn the crank _very_ slowly and notice that the needle of the ammeter swings to and fro with each revolution of the armature. That shows that you have not only a _dynamo_, but an _alternating current_ dynamo. [Illustration: Fig. 6] "Now connect the binding posts _d_ and _e_ of this magneto (Fig. 6) with a short piece of copper wire. Turn the crank and you notice that this dynamo rings two electric bells. Turn slowly and you notice that the alternations of the current are numbered by the strokes on the bells. The hammer swings to and fro just as the needle of the ammeter did. Each bell therefore receives one stroke of the hammer for each revolution of the armature. Now try to turn the crank steadily at the rate of one revolution per second. The armature is making six revolutions, or cycles, per second and you now have not only an alternating current dynamo but a _six-cycle alternating current dynamo_. The lighting circuit used in our apartment is a _sixty_-cycle alternating current. To be sure the armature of the dynamo which generates that current revolves only once a second, but it carries coils enough upon its rim to make that number of alternations.