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Unit systemCGS electromagnetic units
SymbolabC or aC 
Named afterCharles-Augustin de Coulomb
CGS base unitsg1/2?cm1/2
SI units10 C
CGS electrostatic units
Gaussian units

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.


  1. ^ 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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