With reference to Fig. 4.4, we find that
                                              .......................................................(4.4)

Applying Biot - Savart's law for the current element

we can write,

                                           ........................................................(4.5)
Substituting we can write,
                                         .........................(4.6)

We find that, for an infinitely long conductor carrying a current I ,and
Therefore,    .........................................................................................(4.7)