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The abcoulomb (abC or aC) or electromagnetic unit of charge (emu of charge) is the derived physical unit of electric charge in the cgs-emu system of units. One abcoulomb is equal to ten coulombs.

The name abcoulomb was introduced by Kennelly in 1903 as a short name for the long name (absolute) electromagnetic cgs unit of charge that was in use since the adoption of the cgs system in 1875.[1] The abcoulomb was coherent 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; others include CGS-esu, Gaussian units, and Lorentz–Heaviside units. In these other systems, the abcoulomb is not used; CGS-esu and Gaussian units use the statcoulomb is instead, while the Lorentz-Heaviside 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: 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

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

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CGS units
Base units

centimetre gram second

Derived non EM units

barye dyne erg gal kelvin poise phot stilb

Derived EMU units

abampere abcoulomb abhenry abmho abohm abvolt biot gauss gilbert maxwell oersted

Derived ESU units

statcoulomb statmho statohm statvolt

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