During the latter half of the nineteenth century many scientists and engineers were simultaneously observing a strange phenomenon: electrical devices constructed from certain metals seemed to conduct electricity more efficiently in the daytime than at night. This phenomenon, called the photoelectric effect, had been noted years earlier by the French physicist A. E. Becquerel (1820-1891), who had invented a very primitive device for measuring the intensity of light by measuring the elecrical current produced by photochemical reactions. It was becoming evident that one metal in particular--selenium--was far more reactive when exposed to light than any other substance. Using selenium as a base, several scientists set out to develop a practical device for measuring light intensity.

A number of them succeeded. In 1883 the American inventor Charles Fritts created a working photoelectric cell; that same year a German engineer, Paul Nipkow, used a photoelectric cell in his "Nipkow's...