From HandWiki
Short description: Basic unit of electric charge in the CGS-emu system
Unit systemCGS electromagnetic units
SymbolabC or aC 
Named afterCharles-Augustin de Coulomb
1 abC in ...... is equal to ...
   CGS base units   g1/2⋅cm1/2
   SI units   10 C
   CGS electrostatic units   2.997925×1010 statC
   Gaussian units   2.997925×1010 Fr

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 form of (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 Heaviside–Lorentz units. In these other systems, the abcoulomb is not used; CGS-esu and Gaussian units use the statcoulomb 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: 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 accessed 10 December 2020