AN HISTORICAL STUDY 449 
disappeared completely ; the essential identity between currents from all 
sources and from all magnitudes does not seem to have been duly appre- 
ciated. Thus nobody seems to have seen that the large currents of low 
frequency required for synchronisation might be transmitted by the same 
channel as the small currents of high frequency obtained from the photo- 
electric cell. The point that we want to make is that men’s vision was 
limited in both directions by the general outlook of their time. They saw 
very clearly the possibilities of photoelectricity for communication, because 
their thoughts naturally tended in that direction ; but they failed to appre- 
ciate what further elements were required to turn possibility into reality, 
and therefore could not look in the right direction for those elements. 
Meanwhile they failed to think of applications which were wholly or quite 
within their grasp. 
We turn now to the second kind of cell, the emission cell. "The funda- 
mental fact here is that light falling on metals causes them to emit a current 
of electricity, in the form of a stream of electrons ; this can be collected, 
made to pass round an exterior circuit and produce there any electrical 
effect desired. Here we may note a difference between the two kinds of 
cell that may appear at first sight to be important. In the photo-conductive 
cell the energy of the current has to be derived from a battery or some source 
other than the light ; in the emission cell it can be derived from the light 
itself. The emission cell therefore affords in principle the possibility of 
converting sunlight directly into electric power without passing it through 
our present wasteful intermediate stages, such as the growing of vegetation 
which we subsequently burn, or the raising of water to fall and fill rivers. 
But actually nothing has been achieved in that direction even to-day. The 
difficulty is one of mere size. With a cell of given area, we cannot produce 
more power than falls on that area from direct sunlight in the most favourable 
conditions. ‘The amount of power incident on one square yard of the earth’s 
surface in the most favourable conditions is never more than one kilowatt ; 
the average received in our climate is not more than one hundredth of that 
amount. If you work out a sum of proportion you will find that to collect 
the electrical power used in this country, an area at least as large as London 
would have to be covered with photoelectric cells ; and even if we drew our 
power from cloudless regions the cells would occupy the area of a large town. 
Now photoelectric cells are somewhat delicate instruments ; and although 
they need no longer be enclosed in a vacuum, like emission cells, it is quite 
impracticable to make cells of that size. If we tried to use many cells of the 
kind now made, we should require about 5,000 million of them; each 
costs several shillings at least ; and then we should waste most of our power 
in connecting them. No; when we have to harness natural forces, we are 
still forced to resort to nature’s crude and wasteful, but effective, machinery. 
But that is a mere aside ; we must continue the story. The history of 
emission cells, like that of conductive cells, begins with an accident. Hertz 
in 1887, when working on electromagnetic waves, found that the incidence 
of ultra-violet light on a spark gap made it easier for a spark to pass across it. 
Hallwachs in the next year found that the change was due to a current 
flowing from the metal of the gap under the action of the light. He thus 
established the fundamental fact, although of course he did not know that 
the current was carried by a stream of electrons, for electrons were not then 
known ; the effect on which emission cells depend was therefore called the 
Hallwachs effect. But the real parents of emission cells are Elster and 
Geitel who started their work in 1889, and by 1894 had developed the cells 
to a state that remained substantially unchanged for more than thirty years. 
Hallwachs had worked with ordinary metals and had found his effect only 
