As is quite generally known there are certain “semi-metals” which have
the property of changing their resistence to an electric current when
light falls thereon.
Of this group selenium is typical, although there are several others,
thallium, strontium, barium, etc.
More recently it was discovered that some of the rarer alkali metals had
the property, under certain conditions, of actually converting light
into electric current. In this group are potassium, sodium, caesium,
rubidium, etc.
These light sensitive cells vary the electric current quite
accurately in proportion to the intensity of the light falling
thereon, and when available were quickly seized upon by the workers
in “pictures-by-electricity.”
When these light sensitive cells were employed, a modification of the
previous picture-translating methods and mechanisms was made, for now a
modulation instead of an interruption of the electric current was
possible, the modulation representing the values of the halftones of a
picture transparency as well as its blacks and whites.
The rotating cylinder now employed was of glass, around which the
picture, on transparent film, was wrapped. Inside the cylinder a light
was put to shine through the passing picture film as a minute point of
light falling on the light sensitive cell located in a dark box.
Just as in the other cylinder schemes the picture is made to traverse
this point of light until the whole picture is converted into electric
current of corresponding values, which, as before, can be put on a wire,
or can be made to modulate a radio wave.