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 cgs-emu 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.