Imagini ale paginilor
PDF
ePub

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,

[graphic][graphic][ocr errors][ocr errors][subsumed][graphic][subsumed][subsumed][subsumed][merged small]

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.

The apparatus is shown in Fig. 43 on the left of which is the transmitter of the sounds to be sent and on the right is the receiver. The transmitter consists of a vibrating diaphragm D, fitted with a mouthpiece

ட்

S

R

Fig. 43.

like that of a speaking telephone. A beam of light from any source sufficiently bright is concentrated on the diaphragm D by the lens L, and the diaphragm, which is silvered so as to reflect the light, is placed in such a position relatively to the lens L as to project the light along a line joining the axis of another lens L' with the axis of a parabolic reflector R, placed at the distant station and forming part of the receiving apparatus there. The object of the lens L' is to render the divergent rays proceeding from the diaphragm sufficiently parallel as to strike the interior of the reflector R, and be again reflected to a single focus, wherein is placed a small selenium cell s which is electrically sensitive to light. This selenium cell is connected in the circuit of a voltaic battery в and a pair of speaking telephones T T'.

If a person speaks or makes a sound into the mouthpiece of the transmitting apparatus, the diaphragm D

will vibrate in accordance with it, and the vibrating reflector will undulate a beam of light thrown from it, and these undulations travelling along the parallel beam to the reflector at the receiving station, will be directed on the selenium cell, through which the electric current from the battery is flowing. They will consequently vary the internal resistance of the selenium to the passage of the current, and in this way modify the strength of the current flowing through the telephone. As the resistance of selenium falls under the impact of light and falls the more the stronger the light, it follows that the strength of current will vary in proportion to the intensity of the light shed on the cell. In short, the current passing through the telephone will ebb and flow in sympathy with the flickering beam, and therefore with the vibrations of the diaphragm. The result is that the sounds heard in the receiving telephone are an imitation of those uttered at the transmitter. In this way Professor Bell has sent a whisper along a ray of light for several hundred feet. Different forms of photophone, or, to use a more general term, radiophone, have been devised. In some a conical mirror is used as the receiving reflector with good effect. But the chief differences in these lie in the kind of receiving cell employed.

Fig. 44 represents an excellent form of cell devised by M. Mercadier, a French electrician. It consists of two ribbons of brass a b, separated by two ribbons of parchment paper, and rolled into a spiral, then placed been two plates of brass c d, which are connected to the ends a' b', of the two brass ribbons. The brass plates are held between two boards by ties M N, and terminals B A are fitted to these for connect

ing up the cell. The narrow spaces between the edges of the two brass ribbons is then filled up with selenium, put on by heating the point of a selenium pencil until it softens, and drawing it along the surface of the cell. To get good results, it is best to have a battery of ten or fifteen elements on the cells and to employ telephones having a large coil of very fine wire.

M

FIG. 44.

Instead of selenium, tellurium may be used, as was discovered by Professor W. G. Adams. But a simple streak of lamp-black was also found to respond to the light waves by Mr. Summer Tainter, the associate of Professor Bell in his photophonic researches. With lamp-black it is not absolutely necessary to have a telephone and battery in circuit, as the couch of soft carbon itself gives out sounds under the influence of the light, which are audible to an ear held close to it. The effect is feeble but distinct; and is probably due

« ÎnapoiContinuă »