upon its rim to make that number of alternations.

Fig. 7

"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.

Fig. 8

"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.

Fig. 9

"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

Fig. 10

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.