being identical in form and construction; and, as such, its operation will be readily understood. When a sound is produced, energy is expended in its production: this energy cannot be destroyed, although it can seldom be recovered. Ordinarily, when a word is spoken, the energy necessary to or consumed in its utterance first appears as a series of waves of compression and rarefaction in the air, where most of it is finally transformed into heat. It is possible, however, to transmute at least a part of this energy into other forms. Solid bodies may be made to vibrate by the sound of the human voice, and by a suitable contrivance it may be made to do work in running a machine and overcoming other resistances, always, of course, of no great magnitude. In the telephone the sound of the voice is made to do work; this is converted into the energy of an electric current; and this, in turn, is reconverted into mechanical energy, resulting in sound. The form of the ordinary Bell telephone receiver is so well known as hardly to require description.

Internally it consists of a small cylinder of steel which is permanently magnetized, and around one end of which is a coil of fine wire. Just in front of this end of the magnet, but not quite in contact with it, is a thin circular membrane or disk of iron supported at its circum

ference.

Originally the transmitter was precisely like this receiver. One end of the fine wire coil of each is joined to the line connecting the two points, and the other end is connected with the earth.

Now, if a sound be produced near the thin disk of the transmitting instrument, it will be made to vibrate. Although these vibrations

Sectional view of Bell's receiver as now generally used. are exceedingly minute, they are sufficient to produce changes in the magnetic field in which the coil of fine wire lies, as in Page's induction machine; and, as explained in a previous chapter, any change in the nature of this field will produce induced currents in the wire coil. These currents will be transmitted through the line, and, flowing through the coil surrounding the pole of the receiving magnet, will produce variations in the intensity of its magnetization. Just what goes on at the receiving end has been a subject of considerable dispute, and the opera

tion there is unquestionably a complex one. Since sound is produced, there must be vibrations of parts of the receiver; and these must vary in rapidity and form, along with the variations in rapidity and form of the electric waves generated by the action of the transmitting instrument. There is doubtless a vibration of the thin plate of the receiver, due to variations in the strength of the pole of the magnet near which it is placed; but talking can be heard when the metal disk is absent, so a part of the result must be attributed to what is called "molecular " vibration, as in Page's original device for producing sound.

But the wonder of it all is, that the transmitting disk takes on, and the receiving apparatus reproduces, all the various phases and forms of motion impressed upon the air by the voice, and essential to reproduction of that voice in articulate speech. Nothing like it in simplicity of construction, combined with complexity of operation, is to be found in any other

human contrivance.

The electric currents thus generated and transmitted are necessarily extremely minute. The amplitude of vibration of the disk has been estimated to be only a small fraction of the length of a wave of yellow light, of which there are about forty thousand to the inch. It has

also been determined that the receiver reproduces not more than one ten-thousandth part of the "volume of sound" received by the transmitter. As a "motor," it must be considered as having extremely low efficiency, although, on the whole, very effective.

The telephone as at first used, and as just described, was much less satisfactory in its performance than it is at present. Its working has been vastly improved by the use of other forms of transmitting instruments, by means of which variations of current strength of much greater intensity are transmitted over the line, while still retaining the characteristics necessary for the reproduction of speech.

The transmitters generally in use at present depend upon a curious and important discovery made by Hughes in 1878. It consisted essentially in the fact, that, if a piece of carbon be allowed to rest lightly upon another, and an electric current be passed from one to the other in a circuit in which there is a Bell telephone receiver, the latter will respond to the faintest sounds in the vicinity of the carbons. Various other substances (imperfect conductors are generally better) may be used instead of carbon, and the arrangement is called a "microphone,' Its operation is due to the fact that imperfect contact exists where the two carbons touch