Abampere

In physics, the abampere, symbol abA, is the unit of electric current in the cgs-emu (centimeter-gram-second electromagnetic) system of units.

1 abA = 10 A (ampere).

The abampere is not used much anymore, but is needed in some definitions in the Gaussian system of units.

By definition, the abampere is the constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 cm apart in vacuum, would produce between these conductors a force equal to 2 dyne (= 2⋅10⁻⁷ newton) per cm of length in each of the two wires.

This definition is based on ampere's equation in cgs-emu units:

${\frac {F}{l}}={\frac {2i^{2}}{r}},$

where F is the force (in dyne) per length l (in cm) between two infinite long wires in which a current i runs. The distance between the wires is r cm.

The SI definition in meters of one ampere gives a force between the wires that is 100 times larger than the corresponding definition in centimeters. Namely, the factor is 1/100×100² = 100, with the factor 1/100 because of the 100× longer distance r in meters and the factor 100² because of the 100× longer length l in meters. To achieve a force of 2 dyne (the force required in the present centimeter definition as well as in the SI definition), each current must be 10 times larger in the centimeter definition. Hence 1 abA = 10 A.