Part 4

"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!"

"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."

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?"

"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.

"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.

"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.

"Now connect this telephone receiver with the binding posts D and E of this magneto (Fig. 7). Unscrew the cap of the receiver. Move to one side the iron diaphragm and turn slowly the crank of the magneto. Notice that the diaphragm vibrates in time with the alternations of the dynamo. Replace the diaphragm, screw on the cap, hold the receiver to your ear and turn the crank as fast as you can. You will probably be able to make about sixteen cycles per second. The receiver in that case is giving forth a sound of the same pitch as a sixteen-foot closed organ-pipe.

"Connect the telephone receiver to the binding posts D and E of this magneto (Fig. 8), and by means of a belt connect the pulley to this series of cog-wheels. Now you may turn the crank and readily make the armature revolve at the rate of sixty cycles per second, and you notice that you get the same tone that we heard in the dynamo room of the power station and the same tone the telephone receiver gave when I connected it to a coil in our apartment. The tone which is produced by sixty vibrations per second is very nearly that of the C two octaves below middle C on the piano. Try it along with the piano and you will find it a little flat. This string on the piano is making sixty-four vibrations per second.

"Now connect this miniature telephone switchboard lamp with the magneto (Fig. 9) and turn the crank fast. The lamp lights up to full brilliancy and you notice that the light is steady, although it is made by an alternating current passing through the filament in one direction, stopping entirely, and then passing in the opposite direction. The filament has no time to cool off, provided you turn fast enough, but try turning a little slower and you will notice the flickering of the lamp."

III

THE AMMETER

At the last meeting of the Science Club so many questions were asked, which the demonstrators could not answer, that a programme committee, to whom such questions might be referred thereafter, was appointed. It was made the duty of this committee to assign to various members the task of searching for satisfactory answers, and when the material was ready to be reported to the club, the programme committee determined the time and order of presentation. I found that I had been made an honorary member of this committee and that it was expected that I should steer the committee. I told them that I accepted this appointment with the understanding that the fellow who steers is always the smallest man in the crew, and if they would do all the work I would enjoy the honorary title of cockswain. Secretly, however, I appreciated that this was in effect adding several courses to my already rather heavy programme. I must, under the régime, direct a large number of inexperienced students in library research, in laboratory research, and in the art of giving demonstrations with apparatus and experiments to audiences.

The most urgent questions, as also those which were next in the natural order, concerned the ammeter. I told the committee to make that the subject of the next meeting and to send to my laboratory on a certain day the person or persons whom they might appoint to report upon it.

I find that the boys never come singly, but generally in pairs. When the boys came they found lying upon the table an ammeter (Fig. 11).

I told one of them to take out the three screws in the front and remove the face of the instrument. I had told the boys that the instrument cost sixty dollars and that letting them open it was like letting them open my watch. As soon as the face came off one of the boys exclaimed that from my reference to the watch he had expected to see very complicated machinery with many wheels, but from the exceeding simplicity of the mechanism he could not see why it should cost sixty dollars. I told him that although it was a fine piece of workmanship it was fortunately very easy to understand, and I asked them if it reminded them of anything else that they had ever seen. After a few moments of reflection they agreed that it was very much like one of the magnetos. "Well," said I, "where is the field?"

"Is this horseshoe arrangement a magnet?" they inquired.

"There is a compass needle right at your hand waiting to answer that question," I replied. They immediately found that it was a magnet. "Well," I said, "to be really sure that it is a magnet you must find a portion of it that will repel a portion of your compass needle as well as other portions in both horseshoe and needles which attract each other." Whereupon, they found that the portion marked N (Fig. 13) repelled the blue end of the compass needle and attracted strongly the bright end of the needle, while the portion marked S did the reverse. "We will call N and S the poles of the magnet. This is simply a steel bar magnet bent into the shape of a horseshoe."

"You told us," remarked one of the boys, "that steel magnets gradually lose their strength. How then can this be correct as a measuring instrument?"

"It is the purpose of the iron case to enable this magnet to retain its magnetism, and if you will examine its field, as we did that of another magnet upon a former occasion, you will find that although this is a strong steel magnet its field does not extend outside of the iron case. It is as though we could box up magnetism and keep it from escaping.