"Der Lichtelektrische Effekt"

Wilhelm Hallwachs
born 9-Jul-1859, Darmstadt
died 20-Jun-1922, Dresden
1893 Professor of "Electro Technique", and from 1900 Senior Professor in the Department of Physics in the Technische Hochshule, Dresden

a pupil of

Heinrich Herz (right)
born 22-Feb-1857, Hamburg
died 1-Jan-1894, Bonn
Professor of Physics, Karlsruhe and Bonn

-oOo-
Around the time of the centenary of the discovery of the Photoelectric effect - late 1987 - there was some discussion amongst German physicists (particularly those from the Heidelberg area) about whether the general attribution (to Heinrich Herz) was correct or not. A brief outline of some of the early experiments was collated by Dr Hans Schmiedel and exhibited as a poster, along with early X-ray tubes and electrometer detectors, at the ECASIA meeting of that year in Stuttgart. This page is derived in part from that poster.
-oOo-

(The "gypsum" (Gips) shown perhaps could have been a crystal of selenite CaSO₄.2H₂O, which forms thin, transparent, flexible plates with known optical properties. Ed.)

In 1887, almost fifteen years after its publication, the Maxwell Electro-Magnetic Theory of wave propagation was not well known, not much studied, and certainly still required practical confirmation. As part of a systematic investigation into electromagnetic phenomena, Herz (and his students) studied the length of sparks generated by an induction coil (Helmholz, Herz' mentor, had a profound influence on the direction of his investigations). In order to observe the sparks better, Herz darkened the room. Now the spark length changed dramatically - it became smaller. Clearly light had a effect on spark length - illuminated the gap produced long sparks, in darkness only short ones could be generated. In the original observations the light came accidentally from another spark source close by: now the same effect was seen using the (intense) light from burning magnesium. Herz, methodical as always, made a "spectral analysis" and determined that the ultra-violet portion of the magnesium spectrum was the most effective, reporting "Ultraviolettes Licht hatb die Fähigkeit, die Schlagweite eines Induktoriums zu vergrößern" (UV light has the facility to increase to spark length of an induction (apparatus)")

With this in mind and also a sentence from Schuster ("even the smallest electromotive force in a gaseous phase can generate a current..") Hallwachs set up an experiment (Fig.1) After many experimental set-backs, he (Hallwachs) concluded:

when the electroscope plate is negatively charged, then this charge dissipates very quickly under the influence of light.
when the electroscope plate is positively charged, then this charge dissipates very slowly under the influence of light.

Hankel-type electrometer
Stuttgart, 1987

Over the next month a more detailed experimental arrangement was made (Fig. 2) using as detector a sensitive "Hankel Electrometer" Now the effect could be measured, but the measurements didn't resolve much, merely raised new questions:

was the effect really due to the influence of light?
is the root of the effect in the gaseous phase between the light and the detector plate or is it in the solid state nature of the plate itself?
why is the effect unipolar? what is the difference between "the two electricities"?
why does the negative charge go away?

If answers to these points seem obvious with modern hindsight, remember it was then often difficult for experimenters to amass convincing proof for explanations of such unknown and unusual effects using the limited means at their disposal - witness Herz own (flawed) 1892 experimentation leading to assertions of a "non-particulate" nature for the cathode rays, which had to wait another five years for Thomson's correct interpretation (Herz had the right answer (or better a correct answer) for the wrong reasons ...)

These and other questions were addressed during the next three years or so during which time there were some twenty publications on "light electricity". In 1900 there were eight publications against the background of such seminal work as "Die Befreiung der Kathodenstrahlen" ("The liberation of Cathode Rays"), Lennard, 1893, Röntgen's 1895 "New Rays" papers, and the discovery of the Zeemann Phenomenon in 1896.

"Light electricity" had to wait until 1905 for its real explanation - as an aside in the first of Einstein's three great papers of that year - " Über einen die Erzeugung und Verwandlung des Lichtes betreffenden heuristischen Gesichtspunkt" ("Concerning the generation and transformation of light as seen from a heuristic point of view", Annalen der Physik, March 18, 1905)
From then to the time of Siegbahn, many others worked on "light electricity' including Stotlev, Elster, Geitel, Righi, Townsend, Rutherford, Compton and lots of others.

And who did start the whole thing - Herz or Hallwachs?
Herz himself said "Dieses ist erst Herrn Hallwachs gleungen!" ("Hallwachs is the one!")

References
W. Hallwachs, Wiedmann'sche Annalen 29, 1-12, 1886

W. Hallwachs, Wiedmann'sche Annalen 32, 64-74, 1887

H. Hertz, Berliner Berichte 487-490, 1887

H. Hertz, Wiedmann'sche Annalen 31, 983-1000, 1887

Wiedmann, Ebert, Wiedmann'sche Annalen 33, 241- 264, 1887
("Über den Einfluß des Lichtes auf die electrischen Entladungen")

W. Hallwachs, Wiedmann'sche Annalen 33, 301- 312, 1888
("Über den Einfluß des Lichtes auf electrostatisch geladene Körper")

W. Hallwachs, Göttinger Nachrichten 174- 176, 1888
("Über die Electrisierung von Metallplatten mit electrischem licht")

Warburg, Beliner Berichte, 229ff, 1896

W. Hallwachs, "Die Lichtelectrizität", Handbuch der Radiologie, Bd. IIIb, 1913

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