When you speak into the mouth-piece of one telephone, the sound of your voice causes the "diaphragm" to vibrate in front of the magnet. The vibrations cause the magnet's pull upon the diaphragm to vary in force, which variation is answered by electrical waves in the coil and over the wires connected with it. At the other end of the wire the pull of the magnet of the speaking telephone is varied exactly in proportion to the strength of the electric impulses that come over the wire; the varying pull of the magnet sets the diaphragm in motion, and that sets the air in motion in waves precisely like those of the distant voice. When those waves strike the listener's ear, he _seems_ to hear the speaker's exact tones, and so, substantially, he does hear them. The circumstance that electric waves, and not sound-waves, travel over the wires, does not change the quality of the resulting sound in the least. I think you now understand Bell's telephone. The telephones of Edison, Gray, and others, involve different principles and are differently constructed. One invention very often leads to another, and the telephone already has an offspring not less wonderful than itself. It is called the speaking-phonograph. It was invented by Mr. Edison, one of the gentlemen, just mentioned. Evidently, Mr. Edison said to himself: "The telephone hears and speaks; why not make it write in its own way; then its record could be kept, and any time after, the instrument might read aloud its own writing." Like a great genius as he is, Mr. Edison went to work in the simplest way to make the sound-recorder he wanted. You know how the diaphragm of the telephone vibrates when spoken to? Mr. Edison took away from the telephone all except the mouth-piece and the diaphragm, fastened a point of metal, which we will call a "style," to the center of the diaphragm, and then contrived a simple arrangement for making a sheet of tin-foil pass in front of the style. When the diaphragm is still, the style simply scratches a straight line along the foil. When a sound is made, however, and the diaphragm set to vibrating, the mark of the style is not a simple scratch, but an impression varying in depth according to the diaphragm's vibration. And that is how the phonograph writes. To the naked eye, the record of the sound appears to be simply a line of pin points or dots, more or less close to each other; but, under a magnifier, it is seen to be far more complicated.