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AN HISTORICAL STUDY 447 
scientific explanations are irrelevant to our theme. But we must make two 
points clear. The existence of photoelectric cells is no accident ; it arises 
from the most fundamental properties of light and electricity. In the view 
of modern physics light is no longer a discrete agency separable from its 
origins and its effects. To say that light is issuing from a source A and 
falling on a body B is merely a loose way of saying that electrical changes in 
A establish a certain probability that similar electrical changes will occur in 
B. Ina very real sense all actions of light are photoelectric ; the question 
is not how the light that we see can be used to produce electricity ; it is 
rather how the electrical changes that are the primary effect of light come to 
produce vision. Although the sequence of discovery still seems to us 
irregular, we can see now that any prolonged study of light must have led 
to the discovery of photoelectricity, and that by the end of the eighteenth 
century it was already inevitable. 
Again, although all photoelectric effects are in essence the same, super- 
ficially they are different. At the present time four different kinds of 
photoelectric cells are generally distinguished : conductive cells, emission 
cells, voltaic cells, rectifier cells. Each kind has its own advantages and 
limitations ; but to these we shall not often refer; in many fields the 
different kinds are mutually replaceable, at least in principle. Photo-voltaic 
cells are the oldest; the phenomenon that they use was discovered by 
Becquerel in 1839. But the discovery had no practical consequences at the 
time ; and the use that was made of it much later (but then only temporarily) 
was not really based on the original discovery. On the other hand rectifier 
cells were discovered as lately as 1926, when all the principles underlying 
modern applications were already established. They are of very great 
importance, and make some applications much easier ; but they arrived 
too late to affect the main course of photoelectric history, which was 
determined by the discoveries of conductive and emission cells. 
Conductive cells were discovered by Willoughby Smith in 1873. In 
connection with work on telegraph cables, he was seeking a substance of 
high electrical resistance. He thought he had found what he wanted in the 
element selenium, a relatively rare substance resembling sulphur, which 
had been known for many years, but little investigated. However, he found 
that the resistance was very variable, and he soon tracked down the variations 
to changing illuminaton. Selenium when illuminated decreased in resist- 
ance ; the greater the incident light, the greater the current that flowed in 
the circuit containing the selenium resistance. 
The practical implications were realised immediately; and by 1880 
invention was in full swing. In fact the volume of the Electrician for 
1880-1881 devotes a larger proportion of its space to photoelectricity than 
any other volume we have examined. Let us consider what was the state 
of the electrical art at that time. 
The chief industrial use of electricity was still in communication. The 
Electrician gives in each number the prices of telegraph shares, but of no 
others. And there is great excitement over the legal action between the 
Post Office and telephone companies, in which Mr. Justice Stephen decided, 
in the face of expert opinion, that a telephone was a telegraph within the 
meaning of the Act. (Most of the experts are, of course, long dead ; but 
Dr. Fleming, in the guise of Sir Ambrose, is still with us ; we wonder if 
he remembers supporting Stokes in his declaration that there is nothing in 
common between the two instruments and asserting that a telephone was 
nothing but a complicated kind of speaking trumpet!) Of course there 
were other interests; for the foundations of most branches of electrical 
engineering were laid between 1873 and 1880. Arc lamps were actually 
