The Telephone Brought in.
“Observations concerning the effect of light upon the conductivity of selenium had employed the galvanometer solely; it occurred to me that the telephone, from its extreme sensitiveness, might be substituted with advantage. On consideration I saw that the experiments could not be conducted in the ordinary way with continuous light, for a good reason: the law of audibility of the telephone is precisely analogous to the law of electrical induction. No effect is produced during the passage of a continuous and steady current. It is only at the moment of change from a stronger to a weaker state, or, vice versa, that any audible effect is produced; this effect is exactly proportional to the amount of variation in the current. It was, therefore, evident that the telephone could only respond to the effect produced in selenium at the moment of change from light towards darkness, or vice versa, and that it would be advisable to intermit the light with great rapidity so as to produce a succession of changes in the conductivity of the selenium corresponding in frequency to musical vibrations within the limits of the sense of hearing. For I had often noticed that currents of electricity, so feeble as hardly to produce any audible effects from a telephone when the circuit was simply opened and closed, caused very perceptible musical sounds when the circuit was rapidly interrupted; and that the higher the pitch of the sound the more audible was its effect. I was much struck by the idea of producing sound in this way by the action of light. Accordingly I proposed to pass a bright light through one of the orifices in a perforated screen consisting of a circular disk with holes near its circumference. Upon rapidly rotating the disk an intermittent beam of light would fall on the selenium, and from a connected telephone a musical tone would be produced, its pitch depending upon the rapidity with which the disk spun round.