The presence of mercury may cause a selenium cell to be "light negative" i.e., to show an increased resistance in light. There is general agreement that a selenium bridge improves up to a certain point by keeping.
Selenium cells are in use for talking films, where the light through the margin of the film is suitably modified by varying photographic density, so that the current carried through the bridge to a loud-speaker is correspondingly varied and speech reproduced.
To Dr. Fournier d'Albe is due the credit of applying the use of selenium cells for the assistance of blind people. By means of a clockwork device, to interrupt the current through a selenium cell sufficiently rapidly, a musical sound can be heard in a tele phone and the intensity of the note made to serve as a guide to the direction and proximity of a light. Dr. Fournier d'Albe has designed a small portable apparatus by which on pointing it in vari ous directions towards a light, the loudness of the note heard in a telephone indicates brightness. Totally blind persons can find a window in a room or learn to guide themselves about a house by this means. A more important development in this direction however, is his invention of the optophone, an instrument which enables a blind person to read ordinary print by means of sound. This apparatus depends for its action on a selenium cell. It is clear that the pitch of the note, heard in a telephone in circuit with a selenium bridge, can be altered by varying the speed at which perforations in a revolving disc allow light to reach the cell. Different notes may in fact be given out by a loud-speaker con nected with a bridge, and a sort of musical instrument evolved. Similar developments have taken place in America.
Sunshine, fog or moonlight recorders have been made with selenium, and recently in America, the result of a horse race was established by the leading horse intercepting a beam of light and thereby affecting a selenium cell, which actuated a relay, and marked the exact moment of the horse's arrival at the winning post. The variations of light intensity during an eclipse of the
sun have also been recorded by means of selenium.
For photometric work however, for television or all applications where great precision is required, the photo-electric cell with thermionic valve is more reliable at present.
The far greater sensitivity of selenium, and the comparatively large currents it is able to carry, at the same time offers very strong inducement to physicists to continue the study of its re markable properties, quite apart from the intrinsic scientific inter est attaching to the subject.
An ingenious device invented during the War by Prof. A. 0. Rankine, and called the "photophone," enables light reflected from a mirror attached to the needle-holder of a gramophone diaphragm to affect a distant selenium cell and so transmit speech.
Some of the latest applications have been evolved in the labora tories of Messrs. The Radiovisor Parent Ltd., where a system of street lamp control has been perfected so that lamps may be auto matically lighted at dusk and extinguished at dawn. This has now been in use in the urban district of Barnes, London, since March, 1928. A similar method has already been employed in Germany for lighting and extinguishing harbour buoys, etc. Amongst other applications which have been developed in this laboratory are a system of railway control and a burglar alarm.
A device whereby the smoke density in the smoke stacks of destroyers may be registered on an indicator has been experi mented with both in the British and in the U.S.A. Navy. Selenium enables this to be done by recording changes of electrical re sistance when light projected across the lower part of the stack is more or less obscured by the smoke. During the War it was shown to be practicable to regulate the movements of a ship, to fire guns or explode mines by means of selenium bridges con trolled by a searchlight beam.
Two forms of selenium fire alarms have also been invented. Selenium bridges have also been used to increase the speed of telegraphic signalling.