The Weiss magneton was an experimentally derived unit of

magnetic moment In electromagnetism, the magnetic moment or magnetic dipole moment is the combination of strength and orientation of a magnet or other object or system that exerts a magnetic field. The magnetic dipole moment of an object determines the magnitude ...

equal to

joule The joule ( , or ; symbol: J) is the unit of energy in the International System of Units (SI). In terms of SI base units, one joule corresponds to one kilogram- metre squared per second squared One joule is equal to the amount of work d ...

s per tesla, which is about 20% of the

Bohr magneton In atomic physics, the Bohr magneton (symbol ) is a physical constant and the natural unit for expressing the magnetic moment of an electron caused by its orbital or spin angular momentum. In SI units, the Bohr magneton is defined as \mu_\mat ...

. It was suggested in 1911 by Pierre Weiss.

Origin

The idea of elementary magnets originated from the Swiss physicist

Walther Ritz Walther Heinrich Wilhelm Ritz (22 February 1878 – 7 July 1909) was a Swiss theoretical physicist. He is most famous for his work with Johannes Rydberg on the Rydberg–Ritz combination principle. Ritz is also known for the variational method n ...

, who tried to explain atomic spectra. In 1907 he suggested that atoms might contain chains of magnetized and neutral rods, which were the cause of magnetic properties of materials. Just like elementary charges, this was supposed to give rise to discrete values of the total magnetic moment per atom. In 1909, Weiss performed measurements of the saturation magnetization at the temperature of

liquid hydrogen Liquid hydrogen () is the liquid state of the element hydrogen. Hydrogen is found naturally in the molecule, molecular H2 form. To exist as a liquid, H2 must be cooled below its critical point (thermodynamics), critical point of 33 Kelvins, ...

in the laboratory of

Heike Kamerlingh Onnes Heike Kamerlingh Onnes (; 21 September 1853 – 21 February 1926) was a Dutch Experimental physics, experimental physicist. After studying in Groningen and Heidelberg, he became Professor of Experimental Physics at Leiden University, where he tau ...

in Leiden. In 1911, Weiss announced that the molar moments of nickel and iron had the ratio of 3:11, from which he derived the value of a magneton.

Comparisons with early quantum theory

Weiss gave an address about the magneton at a conference in

Karlsruhe Karlsruhe ( ; ; ; South Franconian German, South Franconian: ''Kallsruh'') is the List of cities in Baden-Württemberg by population, third-largest city of the States of Germany, German state of Baden-Württemberg, after its capital Stuttgart a ...

in September 1911. Several theorists commented that the magneton should involve the

Planck constant The Planck constant, or Planck's constant, denoted by h, is a fundamental physical constant of foundational importance in quantum mechanics: a photon's energy is equal to its frequency multiplied by the Planck constant, and the wavelength of a ...

''h''. By postulating that the ratio of electron

kinetic energy In physics, the kinetic energy of an object is the form of energy that it possesses due to its motion. In classical mechanics, the kinetic energy of a non-rotating object of mass ''m'' traveling at a speed ''v'' is \fracmv^2.Resnick, Rober ...

to orbital

frequency Frequency is the number of occurrences of a repeating event per unit of time. Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio ...

should be equal to ''h'',

Richard Gans __NOTOC__ Richard Martin Gans (7 March 1880 – 27 June 1954), German of Jewish origin, born in Hamburg, was the physicist who founded the Physics Institute of the National University of La Plata, Argentina. He was its Director in two different ...

computed a value that was almost an order of magnitude larger than the value obtained by Weiss. At the First Solvay Conference in November that year,

Paul Langevin Paul Langevin (23 January 1872 – 19 December 1946) was a French physicist who developed Langevin dynamics and the Langevin equation. He was one of the founders of the '' Comité de vigilance des intellectuels antifascistes'', an anti-fascist ...

obtained a submultiple which gave better agreement. But once the

old quantum theory The old quantum theory is a collection of results from the years 1900–1925, which predate modern quantum mechanics. The theory was never complete or self-consistent, but was instead a set of heuristic corrections to classical mechanics. The th ...

was a bit better understood, no theoretical argument could be found to justify Weiss's value. In 1920,

Wolfgang Pauli Wolfgang Ernst Pauli ( ; ; 25 April 1900 – 15 December 1958) was an Austrian theoretical physicist and a pioneer of quantum mechanics. In 1945, after having been nominated by Albert Einstein, Pauli received the Nobel Prize in Physics "for the ...

wrote an article where he called the magneton of the experimentalists the Weiss magneton, and the theoretical value the

Further experiments

Despite theoretical problems, Weiss and other experimentalists like Blas Cabrera continued to analyze data in terms of the Weiss magneton until the 1930s.

References

{{reflist Magnetism Obsolete theories in physics