Before the refinement of light sensitive cells, actual electrical
contact was oftenest employed in sending the impulses which represented
light gradations in the picture.
Usually, therefore, a zinc etching of a pen and ink picture was made,
and curved into a cylinder. This picture cylinder was then slipped onto
a rotating mandril, which was also moved axially by a screw thread on
the mandril shaft; or the cylinder rotated and a contact arm moved along
by the screw, like the old wax cylinder phonograph.
A delicately suspended arm, moved along by the screw as just described
and carrying, instead of the phonograph sound box, a very small and
smooth point which was lifted by the high parts of the zinc etched
picture. When so lifted the arm makes electrical contact with an
adjustable point and current is put into the inter-station wire circuit.
By this means the values of the picture are converted into corresponding
duration values of an electric current, and put onto a wire connecting
the sending machine with a distant receiving machine (described in
detail later in the text).
It will thus be seen that the electric impulses sent out over a wire
attached to the contact point represent the value of the light and dark
portions of the picture.
The electric impulses are similar to letter-code dots-and-dashes, for
the picture actually opens and closes the circuit, like a telegraph key,
the dark portions of the picture sending dashes and the light portions
of the picture sending dots. With the point set at one end of the
cylinder, and the contact arm advancing longitudinally by reason of the
thread on the shaft, the point traverses a spiral around the cylinder
until the whole picture is covered.