adjustment and preservation, the grains are sometimes steeped in a limpid oil or spirit. A third form which may be mentioned is the "chain" transmitter, in which the speaking microphone is an iron chain or strip of chain mail kept slightly tense by a spiral spring at one end and attached at the other to a sonorous board. The current traverses the chain, and the joint between every two links is a microphone.

When different metals are employed, as for example, German silver and iron, or bismuth and iron, and the contact points between them are heated by a spirit flame, a thermo-electric current is generated, and a "thermo-microphone" produced. In this case no battery is required, as the microphone supplies its own current; but the effects are of course relatively feeble.

From this and other experimental observations the author has been led to conclude that the action of the microphone is due to a discharge of current through the film of air or liquid in which the metal or carbon is immersed. The more or less uneven surfaces "touch" each other in this film, and the elastic tremors never absent from solid bodies cause the points to separate by an infinitesimal distance, thereby varying the strength of the discharge and causing a feeble sound in the telephone. The more violent tremors caused by the vibrations of speech produce a louder effect by varying the resistance of the fluid film to a greater extent, and modifying the current in accordance with the voice.

The telephone circuit with a microphone transmitter is represented in Fig. 38, where м is the microphone transmitter, and T the telephone receiver, connected

together in circuit with a battery B, the telegraph line L, and the earth-plates E E. The microphone м consists of a pencil of willow carbon c, lightly supported above and below by two little brackets of the same

material, and set on a wooden stand D. The peculiar property of this device is that when waves of sound impinge upon it they shake the loose contact of the pencil c with the brackets, and thereby cause the current flowing from the battery through the microphone into the line to vary in strength according to the undulating waves of the sound. The undulating or sonorous current thus set up traverses the line and passes through the telephone T at the other end. This consists of a coil of fine wire F insulated with silk, and mounted on the pole of a small magnet G. In front of the coil is placed a thin disc of iron H, which is fitted into an ear-piece 1. Now, when the undulating currents flow through the coil to earth they attract more or less, according to their strength, the iron plate н, and thereby set it into a vibration, which is audible to the ear placed at 1, and reproduces the sounds which affected the microphone at the other end,

be these sounds what they may-instrumental music, song, or speech.

The external appearance of the Bell apparatus is shown at T, in Fig. 39, which represents the entire

apparatus fitted up in the room of a user. It includes the alarm-bell в for announcing a conversation, and the microphone transmitter, which is placed within the mouthpiece м, together with a sending battery inside the box. A second telephone of a watch-like pattern is also shown to the left, and can be used for another ear in listening.

There are other forms of Bell's telephone besides that described-for example, the Gower instrument, in which the magnet is made stronger and the disc larger. The Gower-Bell telephone is shown in Fig. 40,

where o is a horse-shoe magnet built up of thin plates of French steel strongly magnetised and carrying on its two poles N s a pair of small bobbins coiled with fine silk-covered copper wire. These bobbins are connected to terminals as shown. Over these bobbins is fixed a sheet iron disc м which vibrates under the

CHAPTER VI.

THE PHOTOPHONE AND TELEPHOTOGRAPH.

A RAY of light is in reality a stream of power; an impalpable shaft conveying energy with inconceivable rapidity; and it has been ingeniously utilised by Professor Graham Bell as the invisible carrier of speech. With the apparatus called the " Photophone" he has succeeded in transmitting telephonic messages both in words and in music along a beam of light in lieu of a metal wire. This achievement reminds us curiously of the fabulous Apollo, god of light and music, and offers a material analogy to inspiration which is very striking.

By the heliograph a series of light signals can be flashed for a distance of one hundred miles or more in a clear atmosphere such as that of Algeria or Afghanistan, each flash of light constituting an element in the letter signalled. But Professor Bell, by a refinement of the heliographic process, has transformed these crude mechanical signals into the delicate undulations of the voice. This he has done by means of his prior invention, the speaking telephone, and the peculiar property possessed by selenium of varying in its electrical resistance under the influence of light.