"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. [Illustration: Fig. 7] "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_. [Illustration: Fig. 8] [Illustration: 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 [Illustration: Fig. 10]