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Lenard

Philipp Eduard Anton Lenard

Achievements: Cathode ray work

Born: June 7, 1862; Pressburg, Austria--Hungary (now Bratislava, Slovakia)

Died: May 20, 1947; Messelhausen, Germany (Aged 84)

Field & Discipline: Science; Physics; Physicist

Language: German

Father: Philipp von Lenard

Mother: Antonie Baumann

Siblings: None

Spouse: Katharina Schlehner (m. 1897)

Children: None

Education: Pressburg Cathedral School (1871-74); Pressburg Realschule (1874-80); Technische Hochschulen in Vienna and Budapest (1880-82); University of Heidelberg (1883-86); University of Berlin (1885-86)

Major Work:

“On Cathode Rays” (1906 )

Life & Times: 

At Heidelberg Lenard studied under [taught by] Bunsen*, and then at the University of Berlin he was taught by the physicist and physiologist [taught by] Helmholtz*,  who decreed his doctoral dissertation subject: on the vibration and electrification of falling raindrops.

During his university vacations Lenard had begun investigations into phosphorescence and discovered that some materials phosphoresced only if they contained trace amounts of certain metals.  By 1889 Lenard had discovered that phosphorescence is caused by the presence of very small quantities of copper, bismuth, or manganese in what were previously thought to be pure alkaline earth sulfides.

In 1889 Lenard spent six months in England, after which he travelled to the University of Breslau (now in Wroclaw, Poland).

In April 1891 he was appointed assistant to [assistant and collaborator with] Hertz*, already famous for his discovery of electromagnetic waves and the photoelectric effect, at the University of Bonn, and here he began working with Hertz on cathode rays.  This subject had first attracted Lenard's attention as early as 1880, when he had read [influenced by] Crookes'* 1879 paper: "Radiant Matter, or the Fourth Physical State."

Utilizing Hertz's discovery that the rays penetrated thin metal sheets, at the end of 1892 Lenard constructed a glass vacuum tube sealed at the positive electrode end by a thin piece of aluminium, which came to be known as a Lenard window.

With this he was then able to direct the rays out of the discharge space and into either open air or a second evacuated space, where they could then be examined independently of the discharge process and were more accessible to experiment.

Using a phosphorescent screen as a detector (it glowed when struck by the rays), Lenard found that the rays emerged for about 8 cm into the air (which became conducting), that the rays decreased in number as the screen was drawn away from the tube and ceased at a distance.

A longer beam of rays was produced in the second vacuum tube,  which could then be deflected with magnetic and electric fields (a phenomenon first discovered by another German physicist, [work related to] Plucker*, in 1858 and then investigated further by [work related to] Hittorf* and [work related to] Goldstein*).

Lenard eventually established (along with [work related to]  J.J. Thomson*) that cathode rays consist of negatively charged particles. During these investigations he was able to measure the ratio of charge to mass of particles, and found that the particles penetrated air and other substances to different depths, that absorption of the rays depended on the thickness and density of the substances, and that the rays were more penetrating at higher tube voltages, corresponding to increasing velocity of the rays.

In January 1894 Hertz died unexpectedly at the age of 36, and Lenard briefly became acting director of the Physical Institute at the University of Bonn. In October 1894 Lenard received his first offer of a tenured professorship, from the University of Breslau, where he remained for a year, before accepting a lectureship at the Technische Hochschule in Aachen, where there was more possibility of research.

In October 1896 he was then offered an extraordinary professorship at the University of Heidelberg, where he remained for two years, before moving in 1898 to the University of Kiel as a full professor and director of the physics laboratory.

Meanwhile, in 1895 [influenced by] Roentgen* had discovered X rays (created by the impact of cathode rays with parts of the discharge tube) which had greatly depressed Lenard, who felt that in the course of his constant experimenting with cathode rays he should have made the discovery himself.

During his time at Kiel, Lenard continued his studies of cathode rays, now greatly aided by the new cathode-ray tube developed by [used device of] Braun* in 1897, and in 1899 he proved that cathode rays are created when ultraviolet light strikes a metal plate  (photoelectricity).

In 1902 Lenard showed that the photoelectric effect produces the same electrons (i.e. with the same charge/mass ratio) found in cathode rays, that the photoelectrons are not just dislodged from the metal surface but ejected with a certain amount of energy depending on the charge of the plate, and that the number of electrons increases with the intensity of radiation, but that their velocities never exceed a certain limit.

These experimental data were explained by [theory proved by] Einstein's* hypothesis of light quanta of 1905, in his application of [theory confirmed by] Planck's* quantum theory.

Also in 1902, Lenard made one of his most significant scientific contributions with the observation that a free electron must have at least a certain energy to ionize a gas by knocking a bound electron out of an atom.

In 1903 Lenard presented his hypothesis that the atom is largely empty space, based on his observations of how electrons passed through a Lenard window and penetrated air and other substances. Proposing that negative and positive electric charges in the atom were arranged tightly in pairs, his model of the atom was an important precursor to that of the  British physicist [anticipated] Rutherford*, who in 1911 demonstrated his atomic model, of a very dense positive nucleus orbited by relatively distant negative electrons.

Lenard was awarded the 1905 Nobel Prize in physics "for his work on cathode rays . . . [which] served in many respects as a basis for the development of the electron theory."

During World War I, Lenard accused the English of systematically hiding and plagiarizing the work of German scientists, and after the war he actively incited his students against the ruling parties of the Weimar Republic, whom he despised for their democratic and antimilitaristic views. He soon became an early supporter of [an early supporter of] Hitler* (whom he met in 1926).

In 1931 Lenard retired as professor of physics at Heidelberg, but with the rise to power of the Nazis in 1933 he was awarded the Eagle Shield of the German Reich and received the title of “Chief of Aryan or German Physics.” Encouraged by the growing acceptance of his views, in 1929 he had published his racially-oriented history of science: “Great Men of Science: A History of Scientific Progress.”

Assessment: None

Oneliner:  PEA Lenard, early pioneer in cathode ray work

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