John Logie Baird (1888-1946)

154

THE WIRELESS WORLD AND RADIO REVIEW

MAY 7, 1924

image of the object to be transmitted,
there was a single selenium cell connected
to a valve amplifier.

In operation the procedure of the mechan-
ism is as follows: The image focussed
on the disc controls the illumination; as
this disc rotates the holes allow light from
each part of the image in succession to fall
on the cell, the light flashes are interrupted

[NLS note: a graphic appears here – see image of page]

by the edge of the serrated disc at a very high frequency before reaching the cell.

Selenium is instantaneous in its response
to light, that is to say, the instant light
falls on it, it begins to change its resistance.
Time, however, is required for this effect
to reach its maximum, and this property
of selenium known as its "chemical inertia,"
or "time lag," is fatal to any system which
depends simply upon passing the image over

a selenium cell, as in passing along a light
strip of the image, the resistance is very
much lower at the end of the strip than at
the beginning, and again in passing along a
dark strip the resistance increases as the
strip is traversed.

The use of the rapidly revolving serrated
disc overcomes this, as the actual resistance
of the cell at any instant is not of consequence

[NLS note: a graphic appears here – see image of page]

- it is the pulsations which are transmitted.

To make the matter clearer it might
be said that light was turned into sound.
Loud for the high lights, low for darker
areas and complete silence for darkness.

The apparatus used for the visible detec-
tion of these impulses as a picture replica
of the original image is in the model illus-
trated on a principle similar to that employed
at the transmitter.