Definition of ampere will be obtained here from the equation of the force between two parallel wires carrying currents.
The force between two parallel wires carrying currents on a segment of length L is
F = (μ0I1I2L) / (2πr)
∴ Force per unit length of the conductor is F/L =(μ0I1I2) / (2πr)
If I1 = I2 = 1A, r = 1m then
F/L =(μ0) / (2π) = 4π x 10-7 / (2π) = 2 x 10-7 Nm-1
The above conditions lead to the following definition of ampere.
definition of ampere
Ampere is defined as that constant current that when flowing through two parallel infinitely long straight conductors of negligible cross-section and placed in air or vacuum at a distance of one meter apart, experiences a force of 2 × 10-7 newton per unit length of the conductor.
[Related: force between two parallel wires carrying currents]