Wilhelm Ludwig Franz Hallwachs (9 July 1859 – 20 June 1922) was a German

physicist A physicist is a scientist who specializes in the field of physics, which encompasses the interactions of matter and energy at all length and time scales in the physical universe. Physicists generally are interested in the root or ultimate cau ...

Life and career

Early years

Hallwachs was born in 1859 in Darmstadt to Ludwig and Emilie Hallwachs. His father was a high ranking public official ( Geheimer Staatsrat) with the Ministry of the Interior and Justice of the Grand Duchy of Hesse, then part of the

German Confederation The German Confederation (german: Deutscher Bund, ) was an association of 39 predominantly German-speaking sovereign states in Central Europe. It was created by the Congress of Vienna in 1815 as a replacement of the former Holy Roman Empire, w ...

Career

From 1878 Hallwachs studied physics, most of his time at the

University of Strasbourg The University of Strasbourg (french: Université de Strasbourg, Unistra) is a public research university located in Strasbourg, Alsace, France, with over 52,000 students and 3,300 researchers. The French university traces its history to the ea ...

, then part of Germany he also did one year at the University of Berlin. At last he was an assistant to August Kundt at Strasbourg where he was awarded a doctorate in 1893. Hallwachs moved on to become an assistant to Friedrich Kohlrausch at the

University of Würzburg The Julius Maximilian University of Würzburg (also referred to as the University of Würzburg, in German ''Julius-Maximilians-Universität Würzburg'') is a public research university in Würzburg, Germany. The University of Würzburg is one of ...

where he stayed there from 1884 until 1886. In 1886, he went to

Leipzig Leipzig ( , ; Upper Saxon: ) is the most populous city in the German state of Saxony. Leipzig's population of 605,407 inhabitants (1.1 million in the larger urban zone) as of 2021 places the city as Germany's eighth most populous, as ...

where he habilitated with

Gustav Heinrich Wiedemann Gustav Heinrich Wiedemann (; 2 October 1826 – 24 March 1899) was a German physicist and scientific author. Life Wiedemann was born in Berlin the son of a merchant who died two years later. Following the death of his mother in 1842 he lived wi ...

until 1888. After that he followed Friedrich Kohlrausch in Strasbourg. Hallwachs became a professor at the

Dresden University of Technology TU Dresden (for german: Technische Universität Dresden, abbreviated as TUD and often wrongly translated as "Dresden University of Technology") is a public research university, the largest institute of higher education in the city of Dresden, th ...

in 1893, originally for electrical engineering until he succeeded August Toepler in 1900 as a professor for physics. In 1921/22 Hallwachs was rector of the university.

Private life

In 1890 Hallwachs married Marie Kohlrausch, the daughter of Professor

Friedrich Kohlrausch (physicist) Friedrich Wilhelm Georg Kohlrausch (14 October 1840 – 17 January 1910) was a German physicist who investigated the conductive properties of electrolytes and contributed to knowledge of their behaviour. He also investigated elasticity, thermo ...

; he was an assistant with her in

Würzburg Würzburg (; Main-Franconian: ) is a city in the region of Franconia in the north of the German state of Bavaria. Würzburg is the administrative seat of the ''Regierungsbezirk'' Lower Franconia. It spans the banks of the Main River. Würzburg ...

Scientific work

Hallwachs was a known as a builder of scientific instruments. Among the devices he invented are the electrometer quadrant and a double refractometer of great precision. Hallwachs was an assistant of

Heinrich Hertz Heinrich Rudolf Hertz ( ; ; 22 February 1857 – 1 January 1894) was a German physicist who first conclusively proved the existence of the electromagnetic waves predicted by James Clerk Maxwell's equations of electromagnetism. The unit ...

, in 1886, before the photoelectric effect was discovered. Hallwachs and Hertz, in 1887, carried on the investigations of electromagnetic waves. In 1888 Hallwachs formulated the hypothesis that a conductive plate on which to focus

ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiation ...

light carries a positive charge because the

electrons The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no ...

are gouged out. This happened with more intensity in selenium. The phenomenon was seen in the same year by A. Righi. The phenomenon was called 'Hallwachs-Effekt', now called the

photoelectric effect The photoelectric effect is the emission of electrons when electromagnetic radiation, such as light, hits a material. Electrons emitted in this manner are called photoelectrons. The phenomenon is studied in condensed matter physics, and solid sta ...

. The investigation of the photoelectric effect laid the foundation for the development of the photoelectric cell, photo electricity and Albert Einstein's quantum light hypothesis.

Works

* The light electricity. In: manual of Radiology, Volume 3, Leipzig 1916, pp. 245–563.

References

{{DEFAULTSORT:Hallwachs, Wilhelm 20th-century German physicists Scientists from Hesse 1922 deaths 1859 births 19th-century German physicists Academic staff of TU Dresden