What is the unit called a gauss?

The unit of magnetic flux density in the centimeter-gram-second electromagnetic system of units, but see below. Symbol, Gs, sometimes G, especially as milligauss, mG. One gauss is a magnetic flux density of 1 maxwell per square centimeter. One gauss corresponds to 10⁻⁴ tesla in SI, although strictly speaking the two units cannot be compared. Usually encountered as milligauss. Sometimes called the abtesla; symbol, abT.

Painting of Gauss

Painted copy by G. Biermann of a portrait by C. A. Jensen (1792-1870) painted in July 1840

The gauss is named for the German mathematician Karl Friedrich Gauss (1777–1855).

The late 19th and early 20th centuries saw more confusion over the meaning of “gauss” than practically any other scientific unit. This confusion arose from the way the cgs electromagnetic system was constructed, having been based on unit magnetic poles. To serve as the phantom fourth dimension that the system needed to be able to include electric and magnetic units, the permeability of free space was set equal to 1 and considered merely a numeric ratio, not a physical quantity. The result was that in the cgs electromagnetic system of units, in free space magnetic field strength (whose conventional symbol is H) and magnetic flux density (symbol, B) are numerically equivalent. The dimensional identity of measures of B and H in the cgs electromagnetic system created the illusion among some workers that B and H were the same. They are, however, different. By analogy, if H is the force that compresses a spring, B is the extent of the compression. Some used the gauss to measure both,¹ some used it only for H and some only for B. These preferences were often national. In the United States after 1900, for example, the gauss was used for B and the gilbert per centimeter for H.

To add to the confusion, in 1895 the British Assn. Committee on Electrical Standards tentatively recommended that the cgs unit of magnetomotive force be called the gauss (see source 2 below).

Further, it is said that those researching the earth’s magnetic field used the gauss as the cgs unit of H, but experts dispute this.²

In 1900, after heated discussion the Fifth International Electrical Congress adopted the name Gauss for the C.G.S. unit of magnetic field (see the report in Nature). It was noted in the discussion that in the United States the “gauss” had been a unit of magnetic field “for some years.”

At its meeting in Stockholm in 1930 the Advisory Committee on Nomenclature of the International Electrotechnical Commission eliminated all ambiguity by adopting the gauss for the unit of magnetic flux density and the oersted for the unit of magnetic field strength.³ (The term oersted had previously been used for the unit of magnetic circuit reluctance.⁴

According to the current national standard in the United States⁵, the gauss is not to be used; the tesla should be used instead. In practice, however, in the early 21^st century the gauss remains the preferred unit in many fields, from astrophysics to magnetic recording technology.

American Institute of Electrical Engineers, Standardization Rules, Rule No. 90; 1915. (provisional)

2. J. H. Dellinger.
International System of Electric and Magnetic Units.
Bulletin of the [U.S.] Bureau of Standards, volume 13, number 4 (March 6, 1917).
See footnote 8 on page 612.

3. International Electrotechnical Commission.
Recommendations in the field of quantities and units used in electricity. (1st ed.)
IEC Publication 164.
Geneva, 1964.

4. The name was suggested by the Committee of Units and Standards of the American Institute of Electrical Engineers in May 1900.

5. IEEE/ASTM SI 10™-2002.
American National Standard for Use of the International System of Units (SI): The Modern Metric System.
New York: IEEE, 30 December 2002.

sources

A unity [sic] of intensity of magnetic field, proposed by Prof. S. P. Thompson. = 10⁸ C.G.S. units. Other values have been suggested.

Latimer Clark.
A Dictionary of Metric and Other Useful Measures.
London: E & F.N. Spon, 1891.

Taking into account the fact that the question of magnetic units is still under discussion by various bodies, the Committee wish to come to no hasty decisions, but they recommend for tentative adoption the following terminology:-

2. That the C.G.S. unit of magnetic potential or of magnetomotive force be called a gauss.

Note.- An ampere-turn corresponds to 4π/10 (or 1.2566) gauss. Hence the number of gausses round any closed curve linked on an electric circuit is equal to 1.2566 times the number of ampere-turns in this circuit.

Twenty-Second Report- Ipswich, 1895.
Reports of the Committee on Electrical Standards Appointed by the British Association for the Advancement of Science.
Cambridge: Cambridge Univ. Press, 1913.
Page 520.

The first thing requiring a name is this quantity magnetic potential, sometimes called magneto-motive furce; a quantity spoken of and measured, not inconveniently but with insufficient generality, by electrical engineers as ampere-turns. It has been proposed (by Mr Heaviside, for instance) that it be called gaussage, and that its C.G.S. unit be one gauss.

Oliver J. Lodge.
Magnetic Units.
Reports of the Committee on Electrical Standards Appointed by the British Association for the Advancement of Science.
Cambridge: Cambridge Univ. Press, 1913.
Page 522.

The American Institute of Electrical Engineers accepted the decision of the Chicago congress and has used the cgs units for magnetic quantities since then. Names were provisionally adopted for the units in 1894. A committee on units and standards recommended the name “gilbert” for the cgs unit of magnetomotive force; “weber” for the cgs unit of magnetic flux; “oersted" for the cgs unit of reluctance; and “gauss” for the cgs unit of magnetic induction. These recommendations were adopted provisionally, i. e., until some suitable authority replaced them. Since then an international congress has given a different name to the unit of flux and has used “gauss” for a different unit. Only two of the 1894 units therefore retained official standing in the United States — the “gilbert” and the “oersted.”

The British Association committee on electrical standards did not follow strictly the Chicago recommendation as to magnetic units. In 1895 the committee recommended for tentative adoption the following two units: The “weber” = 10⁸ cgs units of flux and the “gauss” = cgs unit of magnetomotive force. These two have been superseded by the definitions adopted by the 1900 Paris congress.

At the Paris electrical congress of 1900 the following report was adopted after consideration by section 1 of the congress:

1. The section recommends giving the name “gauss” to the cgs unit of magnetic field intensity.

2. The section recommends giving the name “maxwell” to the cgs unit of magnetic flux.

In its 1911 standardization rules the American Institute of Electrical Engineers adopted the “maxwell” as the cgs unit of magnetic flux, the “gauss” as the cgs unit of magnetic induction, and the “gilbert per centimeter” as the cgs unit of magnetic field intensity. In its 1914 standardization rules the institute added “gauss” as an alternative name for the unit of magnetic field intensity, making the statement: “The gauss is provisionally accepted for the present as the name of both the unit of field intensity and flux density on the assumption that permeability is a simple numeric.” The name “gauss” is applied to both units also by the American Society for Testing Materials. It was so adopted by that society in 1911 in its “Standard tests for magnetic properties of iron and steel.”

The history of magnetic units has thus been a series of conflicting recommendations. The outstanding result of official decisions has been the adoption of the cgs electromagnetic units. The names given the magnetic units have fluctuated and some confusion still exists in the nomenclature.

The name “gauss” is used both for the cgs unit of induction, as defined by the A. I. E. E. in 1894, and for the cgs unit of magnetic field intensity or magnetizing force, as defined by the 1900 Paris congress. This double usage, recently sanctioned by engineering societies, is based upon the mathematical convenience of defining induction and magnetizing force both as the force on a unit magnetic pole in a narrow cavity in the material, the cavity being in the one case perpendicular, and in the other case parallel, to the direction of magnetization. This definition, however, only applies in the ordinary electromagnetic units. There are a number of reasons for considering induction and magnetizing force to be two physically distinct quantities, just as electromotive force and current are physically different. “Gauss” is therefore not as distinctive a name as might be desired, it meaning either cgs unit of induction or cgs unit of magnetizing force. Thus, if the statement is made that the iron in a machine was worked at 100 gausses, one would not know, unless aided by context or convention, whether gausses of induction or magnetizing force were meant.

In the United States “gauss” has been used much more for the cgs unit of induction than for the cgs unit of magnetizing force. It was adopted in the former sense by the A. I. E. E. in 1894 and 1911.

[U. S.] Department of Commerce.
Circular of the Bureau of Standards No. 60.
Electric Units and Standards.
Washington: U.S.G.P.O., 1916.
Page 51.