Combining two scientific discoveries

                             The invention of the photomultiplier is predicated upon two prior achievements, the discoveries of the photoelectric effect and the secondary emission (i.e., the ability of electrons in a vacuum tube to cause the emission of additional electrons by striking an electrode).

Photoelectric effect

The first demonstration of the photoelectric effect was carried out in 1887 by Heinrich Hertz who demonstrated it using ultraviolet light. Significant for practical applications, Elster and Geitel two years later demonstrated the same effect using visible light striking alkali metals (potassium and sodium). The addition of caesium, another alkali metal, has permitted the range of sensitive wavelengths to be extended towards longer wavelengths in the red portion of the visible spectrum.

Historically, the photoelectric effect is associated with Albert Einstein, who relied upon the phenomenon to establish the fundamental principle of quantum mechanics, in 1905, an accomplishment for which Einstein received the 1921 Nobel Prize. It is worthwhile to note that Heinrich Hertz, working 18 years earlier, had not recognized that the kinetic energy of the emitted electrons is proportional to the frequency but independent of the optical intensity. This fact implied a discrete nature of light, i.e. the existence of quanta, for the first time.

Secondary emission

The phenomenon of secondary emission was first limited to purely electronic inventions (i.e., those lacking photosensitivity). In 1902, Austin and Starke reported that the metal surfaces impacted by electron beams emitted a larger number of electrons than were incident. The application of the newly discovered secondary emission to the amplification of signals was only proposed after World War I by Westinghouse scientist Joseph Slepian in a 1919 patent.