Excessive illumination will also produce similar results. The inertia of a cell is practically unaffected by the wavelength of the light used, but the maximum sensitiveness of a cell is towards the yellow-orange portion of the spectrum.
In addition to light, heat has also been found to vary the electrical resistance of selenium in a very remarkable manner. At 80° C. selenium is a non-conductor, but up to 210° C. the conductivity gradually increases, after which it again diminishes.
APPENDIX B
PREPARING THE METAL PRINTS
Electricians who desire to experiment in photo-telegraphy, but who have no knowledge of photography, may perhaps find the following detailed description of preparing the metal prints of some value. The would-be experimenter may feel somewhat alarmed at the amount of work entailed, but once the various operations are thoroughly grasped, and with a little patience and practice, no very great difficulty should be experienced. The simpler photographic operations, such as developing, fixing, etc., cannot be described here, and the beginner is advised to study a good text-book on the subject.
The method to be given of preparing the photographs is practically the only one available for wireless transmission, and although the manner given of preparing is perhaps not strictly professional, having been modified in order to meet the requirements of the ordinary amateur experimenter, the results obtained will be found perfectly satisfactory.
As will have been gathered from Chapter II., the camera used for copying has to have a single line screen placed a certain distance in front of the photographic plate, and the object of this screen is to break the image up into parallel bands, each band varying in width according to the density of the photograph from which it has been prepared. Thus a white portion of the photograph would consist of very narrow lines wide apart, while a dark portion would be made up of wide lines close together; a black part would appear solid and show no lines at all. It is, of course, obvious
that the lines on the negative cannot be wider apart, centre to centre, than the lines of the screen. A good screen distance has been found to be 1 to 64, i.e. the diameter of the stop is 1/64th of the camera extension, and the distance of the screen lines from the photographic plate is 64 times the size of the screen opening. The following table shows what this distance is for the screen most likely to be used. The line screens used consist of glass plates upon which a number of lines are accurately ruled, the width of the lines and the spaces between being equal; the lines are filled in with an opaque substance. These ruled screens are very expensive, and are only made to order,[[10]] a screen half-plate size costing from 21s. to 27s. 6d. An efficient substitute for a ruled screen can be made by taking a rather large sheet of Bristol board and ruling lines across in pure black drawing ink, the width of the lines and the spaces between being 1/12th of an inch respectively. A photograph must be taken of this card, the reduction in size determining the number of lines to the inch. A card 20 × 15 inches, with 12 lines to the inch, would, if reduced to 5 × 4 inches, make a screen having 48 lines to the inch. Preparing the board is rather a tedious operation, but the line negative produced will be found to give results almost as good as those obtained from a purchased screen.