THE PHOTOPHONE.

Daily Telegraph (Napier), Issue 2978, 11 January 1881, Page 4

THE PHOTOPHONE.

During his recent visit to Paris, Professor Graham "Bell carried out a number of interesting experiments with this instrument, and under somewhat different conditions to those under which ho had operated in the United States. The principal of these was the substitution of tho electric light for that of tho sun, which last in Paris at that time of the year is nearly as uncertain as it is in this country. The photophono may be briefly described as an apparatus constructed to convey sounds at a distance by the intervention of a beam of light. It includes an ordinary telephone. The receiver of the light, however, is of peculiar construction, is especially sensitive to its action, and is, in fact, a microphone. A beam of light, from whatever source (in the present case a Gramme machine and a Duboscq lamp), after being concentrated by a lens, falls upon a small mirror, whence it is reflected into the mouth of a large parabolic silvered reflector. In the focus of this reflector is placed the receiver above referred to. In appearance it is like a small electric coil, or rather a solenoid, about sin. long and nearly 2in. in diameter. This receiver is composed a number of thin discs of tin and mica, placed alternately, the whole being pressed together by metal ends with small tie-rods connecting them. The discs of mica are of less diameter than those of tin, this deficiency of diameter being made up by means of a thin layer of selenium, with which these cavities are filled up. Consequently it is only through the selenium that the tin plates have any electric communication with each other. To render this current a' quantity' of all the even discs of tin are con nected together, as is likewise the case with the odd ones. A contact screw at each end of the apparatus allows this selenium re ueiver to be placed in an electric circuit.

In tbe experiments in question a battery of fifteen Leclaneho cells was generally used. The ordinary telephonic receivers were placed some forty yards away, several rooms intervening. The small mirror upon which the beam of light is, in the first instance, allowed to fall, is so thin as to be flexible, and susceptible of alteration of convexity under the influence of the vibrations caused by different sounds. Into the back of the mirror case is fixed a flexible speaking tube, and by this means these vibratory differences make themselves felt. This mirror, in the present instance, consists of a very thin glass disc, about two inches in diameter, and not more than l-200th of an inch in thickness. Tho surface of this small mirror becomes convex and concave under the effect of the different sound vibrations. These variations in its foim act upon the rays of light reflected from its surface, causing them to disperse or to concentrate ; so that their intensity, when by means of the parabolic reflector they fall upon the selenium receiver is always varying. These variations, in direct response to those of the sound vibrations, are transmitted fceicphonically to tbe ordinary receiver*. l at ihe other end, where the sounds are reproduced. Certain sounds and words, especially those containing gutturals, were heard much more distinctly than oth.r?. Tho distance along which' the vibratr ■■_■. ■■ ..y of light travelled—that is, from I • . !:;; H flexible mirror to the selenium receive —v- is. in the present instance, about 10ft, If, in-Lead of the above experiment, a rotating perforated disc was placed in the focus of the beam of light between the collecting mirror and the selenium receiver, a musical note was reduced, the exact pitch of which depended upon the rapidity of rotation of the disc. This musical note, and consequently its transmission, could be interpreted at will by the interposition of the band, or any other solid substance on the mirror side of the rotation disc. The experiments were, on the whole, highly successful in demonstrating the transmission of sound by means of the " photophono."— Times,