The abcoulomb (abC or aC) or electromagnetic unit of charge (emu of charge) is the derived

physical unit A unit of measurement, or unit of measure, is a definite magnitude of a quantity, defined and adopted by convention or by law, that is used as a standard for measurement of the same kind of quantity. Any other quantity of that kind can ...

electric charge Electric charge (symbol ''q'', sometimes ''Q'') is a physical property of matter that causes it to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative''. Like charges repel each other and ...

in the cgs-emu system of units. One abcoulomb is equal to ten

coulomb The coulomb (symbol: C) is the unit of electric charge in the International System of Units (SI). It is defined to be equal to the electric charge delivered by a 1 ampere current in 1 second, with the elementary charge ''e'' as a defining c ...

s. The name "abcoulomb" was introduced by Kennelly in 1903 as a short form of ''(absolute) electromagnetic cgs unit of charge'' that was in use since the adoption of the cgs system in 1875.A.E. Kennelly (1903
"Magnetic units and other subjects that might occupy attention at the next international electrical congress"
''20th Annual Convention of the American Institute of Electrical Engineers, 1903'' accessed 10 December 2020 The abcoulomb was

coherent Coherence is, in general, a state or situation in which all the parts or ideas fit together well so that they form a united whole. More specifically, coherence, coherency, or coherent may refer to the following: Physics * Coherence (physics ...

with the cgs-emu system, in contrast to the coulomb, the practical unit of charge that had been adopted too in 1875. CGS-emu (or "electromagnetic cgs") units are one of several systems of electromagnetic units within the

centimetre gram second system of units upright=1.35, Different lengths as in respect to the electromagnetic spectrum, measured by the metre and its derived scales. The microwave is in-between 1 meter to 1 millimeter. A centimetre (International spelling) or centimeter (American ...

; others include CGS-esu,

Gaussian units Gaussian units constitute a metric system of units of measurement. This system is the most common of the several electromagnetic unit systems based on the centimetre–gram–second system of units (CGS). It is also called the Gaussian unit syst ...

, and

Heaviside–Lorentz units Heaviside–Lorentz units (or Lorentz–Heaviside units) constitute a system of units and quantities that extends the CGS with a particular set of equations that defines electromagnetic quantities, named for Oliver Heaviside and Hendrik Antoon L ...

. In these other systems, the abcoulomb is not used; CGS-esu and Gaussian units use the

statcoulomb The statcoulomb (statC), franklin (Fr), or electrostatic unit of charge (esu) is the unit of measurement for electrical charge used in the centimetre–gram–second electrostatic units variant (CGS-ESU) and Gaussian systems of units. In term ...

instead, while the Heaviside–Lorentz unit of charge has no specific name. In the electromagnetic cgs system, electric current is a fundamental quantity defined via Ampère's law and takes the permeability as a dimensionless quantity (relative permeability) whose value in a vacuum is unity. As a consequence, the square of the speed of light appears explicitly in some of the equations interrelating quantities in this system. The definition of the abcoulomb follows from that of the

abampere The abampere (abA), also called the biot (Bi) after Jean-Baptiste Biot, is the derived electromagnetic unit of electric current in the emu-cgs system of units (electromagnetic cgs). One abampere corresponds to ten amperes in the SI system of u ...

: given two parallel currents of one abampere separated by one centimetre, the force per distance of wire is 2 dyn/cm. The abcoulomb is the charge flowing in 1 second given a current of 1 abampere.

References

Units of electrical charge Centimetre–gram–second system of units {{Measurement-stub