= 250,000 ohms
can be reduced to 60,000 ohms from the light of a 16 c.p. lamp placed only
a short distance away; the resistance may be still {114} further decreased
by continuing the illumination, but this produces a permanent defect in the
cells termed "fatigue," the cells becoming very sluggish in their action
and their sensitiveness gradually becoming less, the ratio between their
resistance in the dark and their resistance when illuminated being reduced
by as much as 30 per cent.

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 80deg C. selenium is
a non-conductor, but up to 210deg C. the conductivity gradually increases,
after which it again diminishes.

* * * * *

{115}

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,