Ampere is defined as that constant current which when flowing through two parallel infinitely long straight conductors of negligible cross section and placed in air or vacuum at a distance of one metre apart, experience a force of 2 × 10-7 newton per unit length of the conductor.
The force between two parallel wires carrying currents on a segment of length l is
F = (μ0I1 I2l / 2πα)
Force per unit length of the conductor is
F/l = (μ0I1 I2/ 2πα)
If I2 = I2 = 1A, a = 1m
F/l = μ0 /2π (1*1 / 1) = (4π * 10-7) / 2π
= 2 x 10-7 Nm-1
The above conditions lead the definition of ampere.