380 THE QUANTUM THEORY
It was not an electrostatic effect, since it made no qualitative difference whether
the interposed surface was a conductor or an insulator. Hertz began to suspect
that it might be due to the light given off by the primary spark. In a delightful
series of experiments, he confirmed his guess: light can produce sparks. For exam-
ple, he increased the distance between the metal surfaces until sparks ceased to be
produced. Then he illuminated the surfaces with a nearby electric arc lamp: the
sparks reappeared. He also came to the (not quite correct) conclusion that 'If the
observed phenomenon is indeed an action of light, then it is only one of ultraviolet
light.'
1888: Hallwachs. Stimulated by Hertz's work, Wilhelm Hallwachs showed
next that irradiation with ultraviolet light causes uncharged metallic bodies to
acquire a positive charge [H4].
The earliest speculations on the nature of the effect predate the discovery of the
electron in 1897. It was suggested in 1889 that ultraviolet light might cause specks
of metallic dust to leave the metal surface [ L4].
1899: J. J. Thomson. Thomson was the first to state that the photoeffect
induced by ultraviolet light consists of the emission of electrons [T3]. He began
his photoelectric studies by measuring the e/m of the particles produced by light,
using the same method he had applied to cathode rays two years earlier (the par-
ticle beams move through crossed electric and magnetic fields). His conclusion:
'The value of m/e in the case of ultraviolet light. ... is the same as for cathode
rays.' In 1897 he had been unable to determine m or e separately for cathode
rays. Now he saw his way clear to do this for photoelectrons. His second conclu-
sion: 'e is the same in magnitude as the charge carried by the hydrogen atom in
the electrolysis of solutions.'
Thomson's method for finding e is of major interest, since it is one of the earliest
applications of cloud chamber techniques. His student Charles Thomson Rees
Wilson had discovered that charged particles can form nuclei for condensation of
supersaturated water vapor. Thomson applied this method to the determination
of the number of charged particles by droplet counting. Their total charge was
determined electrometrically. In view of these technical innovations, his value for
e (6.8 X 10~^10 esu) must be considered very respectable.
1902: Lenard. In 1902 Philip Lenard studied the photoeffect using a carbon
arc light as a source. He could vary the intensity of his light source by a factor of
- He made the crucial discovery that the electron energy showed 'not the
slightest dependence on the light intensity' [L5]. What about the variation of the
photoelectron energy with the light frequency? One increases with the other; noth-
ing more was known in 1905 [S2].
1905: Einstein. On the basis of his heuristic principle, Einstein proposed the
following 'simplest picture' for the photoeffect. A light-quantum gives all its
energy to a single electron, and the energy transfer by one light-quantum is inde-
pendent of the presence of other light-quanta. He also noted that an electron
ejected from the interior of the body will in general suffer an energy loss before
