The ampere has been defined (by the International Committee of Weights and Measures in 1946) as the current flowing in two long parallel wires 1 metre apart that produces a force of 2 x 10-7 Newtonsper metre length between the wires.
So with the same current flowing in both wires spaced 1 metre apart, it is easy to increase the current until a force of 2 x 10^{-7} Newtons is measured between them. We then know that the current flowing is exactly 1 amp (by definition). This definition also gives the value of k = 2 x 10^{-7} Newtons.metre/amp
If we express one of the currents in terms of the magnetic field that it produces, then we obtain an equation for the force per metre lengthexperienced by a current carrying conductor in a magnetic field. This is by far the most useful form of the equation in practice:
Since k = 2 x 10^{-7} then we write the new constant as µ_{0 }where
µ_{0} = 4π x 10-7 Henrys/metre
(note the Henry is a unit of measure of inductance
µ_{0} is known as the magnetic permeability of free space