450 PHOTOELECTRICITY, ART AND POLITICS 
with ultra-violet light. Elster and Geitel showed that visible light would 
produce the same effect in the ‘ alkali ’ metals, sodium and potassium, which 
can exist in the metallic state only if preserved from contact with the air. 
They devised methods of handling the metals in a vacuum—for all our 
modern vacuum technique is long subsequent to their work ; they investi- 
gated with great care the relation between light and current with a practical 
purpose that we shall notice later, and they invented the ‘ gasfilled ’ cell, 
in which the primary photoelectric current is amplified by passing through 
an inert gas. Their work was magnificent; how magnificent only those 
know who, after a generation, have tried to follow in their path. Even 
the last ten years, when interest has been world-wide and inspired by 
vastly fuller knowledge, have added only a few inessential details to their 
achievement. 
And yet it remained almost unnoticed. Unlike the far less thorough 
work of Willoughby Smith and his immediate successors, it produced no 
spate of invention. It was quite unnoticed by engineers. For thirty years 
engineers, if they thought about photoelectricity at all, thought of it in 
terms of the selenium cell ; and even to-day many partially informed people 
imagine that all photoelectric cells contain selenium. There was perhaps 
some excuse. For the currents obtainable from emission cells are markedly 
smaller than those obtainable from conductive cells ; and it was the smallness 
of even these currents which stood in the way of their application. But the 
real reason was one of atmosphere. 
Elster and Geitel were physicists, not engineers. Their cells were born 
in the atmosphere of ‘ pure science "—an unfortunate term, but there is 
none better. And as the seventies were the great period of electrical 
engineering, so the nineties were the great period of pure science. Modern 
physics, a wholly new science, was born in that decade. It is usually dated 
from RG6ntgen’s discovery of X-rays in 1895, which led to the study of the 
electrical properties of gases, to the discovery of the electron, to radio- 
activity, and to all of that vast field of new facts concerning the interior of 
the atoms which finally led in our own day to the revolutionary theories by 
which alone they can be explained. In that great advance the Hallwachs 
effect has played an essential part. [he facts on which emission cells rely 
were probably the most powerful arguments for rejecting the mechanical 
theories of the older physics and for accepting the bewildering ideas of 
quantum theories. The theoretical implications of this branch of photo- 
electricity completely overshadowed its practical potentialities ; emission 
cells were regarded as laboratory curiosities, productive of nothing but a 
welter of contradictory philosophies, no concern of a self-respecting engineer! 
Ten years ago, fifty years after the dawn of photoelectricity, its sun had 
apparently set. Photoelectric cells were being used to some small extent 
in laboratories and observatories ; but elsewhere, although interest in it 
had never really ceased, its prospects were very dim. Photoelectricity had 
appeared before the world was ready for it. Inventors had lost heart 
because they had tried to run before they could walk, and because they had 
failed to keep abreast of knowledge outside their immediate view. 
And then the unexpected happened, as it always does. In 1926 talking 
films were issued from Hollywood. They took a little longer to reach 
England. ‘The first joke about them we have found in Punch—that invalu- 
able record of our social history—occurs at the end of 1928 ; it is difficult to 
realise how recent they are. Now there was nothing new in the idea of 
associating sound with the moving picture; it had been proposed and 
actually achieved in quite the early days. For the technical means were 
already to hand in the gramophone, which, of course, antedates the moving 
