Jean-Baptiste Biot and Felix Savart jointly gave the Biot-Savart’s law in the year 1820. This is a rule or law which helps to calculate the magnitude of the magnetic field due to a current carrying conductor.

Let ‘dl’ be a current element of a conductor ‘XY’ carrying current ‘I’. Let ‘P’ be a point at a distance ‘r’ from ‘dl’, where the magnetic field is to be determined. Let θ be the angle between ‘dl’ and ‘r’.

Now, according to Biot-Savart’s law, the elementary magnetic field ‘dB’ at the point ‘P’ due to current element ‘dl’ is:

i) directly proportional to magnitude of current passing through it, i.e. dB ∝ I …. (i)

ii) directly proportional to the length of current element, i.e. dB ∝ dl …. (ii)

iii) directly proportional to sine angle between ‘dl’ and ‘r’, i.e. dB ∝ sinθ …. (iii)

iv) inversely proportional to square of the distance between ‘dl’ and ‘P’, i.e. dB ∝ ^{1}/_{r2} …. (iv)

Now, combining the above given relations (i), (ii), (iii) and (iv), we get,

dB ∝ ^{Idlsinθ}/_{r2}

∴ dB = ^{kIdlsinθ}/_{r2}, where k is proportionality constant.

In S.I. system,

k = ^{μo}/_{4π}, where μ_{o} = 4π × 10^{-7} H/m.

In C.G.S. system, k=1. So,

dB = ^{μo}/_{4π}×^{Idlsinθ}/_{r2} …. (v)

∴ This is the mathematical form of Biot-Savart’s law.

The total magnetic field due to the whole conductor is obtained by integrating the equation (v),

∴ B =∫dB

or, B = ∫^{μIdlsinθ}/_{4πr2}

∴ B = ^{μo}/_{4π}∫^{Idlsinθ}/_{r2}