THE LIGHT-QUANTUM 379
19e. The Photoelectric Effect: The Second Coming of h
The most widely remembered part of Einstein's March paper deals with his inter-
pretation of the photoelectric effect. The present discussion of this subject is orga-
nized as follows. After a few general remarks, I sketch its history from 1887 to
- Then I turn to Einstein's contribution. Finally I outline the developments
up to 1916, by which time Einstein's predictions were confirmed.
These days, photoelectron spectroscopy is a giant field of research with its own
journals. Gases, liquids, and solids are being investigated. Applications range from
solid state physics to biology. The field has split into subdisciplines, such as the
spectroscopy in the ultraviolet and in the X-ray region. In 1905, however, the
subject was still in its infancy. We have a detailed picture of the status of photo-
electricity a few months before Einstein finished his paper on light-quanta: the
first review article on the photoelectric effect, completed in December 1904 [S2],
shows that at that time photoelectricity was as much a frontier subject as were
radioactivity, cathode ray physics, and (to a slightly lesser extent) the study of
Hertzian waves.
In 1905 the status of experimental techniques was still rudimentary in all these
areas; yet in each of them initial discoveries of great importance had already been
made. Not suprisingly, an experimentalist mainly active in one of these areas
would also work in some of the others. Thus Hertz, the first to observe a photo-
electric phenomenon (if we consider only the so-called external photoelectric
effect), made this discovery at about the same time he demonstrated the electro-
magnetic nature of light. The high school teachers Julius Elster and Hans Geitel
pioneered the study of photoelectric effects in vacuum tubes and constructed the
first phototubes [E9]; they also performed fundamental experiments in radioac-
tivity. Pierre Curie and one of his co-workers were the first to discover that pho-
toelectric effects can be induced by X-rays [Cl]. J. J. Thomson is best remem-
bered for his discovery of the electron in his study of cathode rays [T2]; yet
perhaps his finest experimental contribution deals with the photoeffect.
Let us now turn to the work of the pioneers.
1887: Hertz. Five experimental observations made within the span of one
decade largely shaped the physics of the twentieth century. In order of appearance,
they are the discoveries of the photoelectric effect, X-rays, radioactivity, the Zee-
man effect, and the electron. The first three of these were made accidentally. Hertz
found the photoeffect when he became intrigued by a side effect he had observed
in the course of his investigations on the electromagnetic wave nature of light
[H3]. At one point, he was studying spark discharges generated by potential dif-
ferences between two metal surfaces. A primary spark coming from one surface
generates a secondary spark on the other. Since the latter was harder to see, Hertz
built an enclosure around it to eliminate stray light. He was struck by the fact
that this caused a shortening of the secondary spark. He found next that this effect
was due to that part of the enclosure that was interposed between the two sparks.
