The Weiss magneton was an experimentally derived unit of magnetic moment

Magnetic moment

The magnetic moment of a magnet is a quantity that determines the force that the magnet can exert on electric currents and the torque that a magnetic field will exert on it...

 equal to joule

Joule

The joule ; symbol J) is a derived unit of energy or work in the International System of Units. It is equal to the energy expended in applying a force of one newton through a distance of one metre , or in passing an electric current of one ampere through a resistance of one ohm for one second...

s per tesla

Tesla (unit)

The tesla is the SI derived unit of magnetic field B . One tesla is equal to one weber per square meter, and it was defined in 1960 in honour of the inventor, physicist, and electrical engineer Nikola Tesla...

, which is about 20% of the Bohr magneton. It was suggested in 1911 by Pierre Weiss

Pierre Weiss

Pierre-Ernest Weiss was a French physicist who developed the domain theory of ferromagnetism in 1907. Weiss domains and the Weiss magneton are named after him. Weiss also developed the Molecular or Mean field theory, which is often called Weiss-mean-field theory.Weiss was born in Mulhouse and...

.

Origin

The idea of elementary magnets originated from the Swiss physicist Walther Ritz, 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

Saturation (magnetic)

Seen in some magnetic materials, saturation is the state reached when an increase in applied external magnetizing field H cannot increase the magnetization of the material further, so the total magnetic field B levels off...

 at the temperature of liquid hydrogen

Liquid hydrogen

Liquid hydrogen is the liquid state of the element hydrogen. Hydrogen is found naturally in the molecular H2 form.To exist as a liquid, H2 must be pressurized above and cooled below hydrogen's Critical point. However, for hydrogen to be in a full liquid state without boiling off, it needs to be...

 in the laboratory of Heike Kamerlingh Onnes

Heike Kamerlingh Onnes

Heike Kamerlingh Onnes was a Dutch physicist and Nobel laureate. He pioneered refrigeration techniques, and he explored how materials behaved when cooled to nearly absolute zero. He was the first to liquify helium...

 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

The City of Karlsruhe is a city in the southwest of Germany, in the state of Baden-Württemberg, located near the French-German border.Karlsruhe was founded in 1715 as Karlsruhe Palace, when Germany was a series of principalities and city states...

 in September 1911. Several theorists commented that the magneton should involve Planck's constant h. By postulating that the ratio of electron kinetic energy

Kinetic energy

The kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...

 to orbital frequency

Frequency

Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...

 should be equal to h, Richard Gans

Richard Gans

Richard Martin Gans , German of Jewish origin, born in Hamburg, was the physicist who founded the Physics Institute of the National University of La Plata, Argentina...

 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

Paul Langevin was a prominent 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 antifascist organization created in the wake of the 6 February 1934 far right riots...

 obtained a submultiple which gave better agreement. But once the old quantum theory

Old quantum theory

The old quantum theory was a collection of results from the years 1900–1925 which predate modern quantum mechanics. The theory was never complete or self-consistent, but was a collection of heuristic prescriptions which are now understood to be the first quantum corrections to classical mechanics...

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

Wolfgang Pauli

Wolfgang Ernst Pauli was an Austrian theoretical physicist and one of the pioneers of quantum physics. In 1945, after being nominated by Albert Einstein, he received the Nobel Prize in Physics for his "decisive contribution through his discovery of a new law of Nature, the exclusion principle or...

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

Further experiments

Despite theoretical problems, Weiss and other experimentalists like Blas Cabrera

Blas Cabrera Felipe

Blas Cabrera y Felipe was a Spanish physicist.-Biography:Cabrera received his baccalaureate in La Laguna . He then moved to Madrid where he began studying law, following family tradition. He met at that time Santiago Ramón y Cajal, who convinced him to abandon law and study science...

continued to analyze data in terms of the Weiss magneton until the 1930's.