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.
