The Electronics Revolution Inventing the Future

26 Seeing by Electricity: Development of Television
The Elster-Geitel photocell was a simple device. In a glass envelope was a curved cathode
of a film of sodium potassium alloy and a metal anode. With a voltage across the
device no current would flow when it was in the dark, but the current would increase when
light shone onto it. Further research showed that alkali metals further up the periodic table,
such as rubidium and caesium, gave even better results. The great advantage of the photocell
was that it responded almost instantly to changes of light compared with the sluggish
response of the selenium sensor.
Once into the twentieth century it seemed likely that a serious attempt would be made
to produce a practicable television system. It was a Russian of Swedish descent called
Boris Rosing (Russians feature strongly in this story) who was the first, though he talked
about ‘electrical telescopy’ in his 1907 patent. 8 He didn’t use a Nipkow disk as the scanning
arrangement in his ‘camera’ but a couple of multifaceted mirrors at right angles to
each other and a photocell as the detector.
The receiver was a form of Braun’s tube (see Chap. 2). The brightness of the spot was
controlled from the output of the photocell, and the horizontal and vertical scans from signals
generated by magnets and coils attached to the rotating mirrors. By 1911 Rosing was
able to demonstrate a practical device. The disadvantage was that it needed three signals
between the transmitter and the receiver, but this wasn’t too great a drawback if it was used
for what he envisaged, which is what we would now call closed circuit television (Fig. 4.1).
In practice, Rosing’s device could do little more than transmit silhouette images, but at
40 pictures a second it was faster than the 12 images a second that had been found necessary
to give a reasonable sense of motion. 9 Though it was recognized at the time that it
wasn’t a complete solution, it was a useful step along the way. Rosing had understood the
need for a receiver that could respond rapidly and that an electronic solution was better
than a mechanical one. What he didn’t know was how to produce a transmitter following
the same principle.
Almost at the same time there was a man in Britain who thought he knew the answer. He
was A. A. Campbell Swinton, a consultant in London who had helped Marconi on his way.
As his contribution to a discussion on this subject he proposed that ‘kathode rays’, as he
called them, should be employed at both the transmitter and receiver. 10 Some form of Braun’s
Fig. 4.1 (a) Rosing’s mirror drum transmitter and (b) his Braun tube receiver. Source:
Scientific American 23 Dec 1911