State Biot-Savart's law.

Explanation

The magnitude of magnetic field `dvecB` at a point P at a distance r from the small elemental length taken on a conductor carrying current varies
(i) directly as the strength of the current I
(ii) directly as the magnitude of the length element `dvecl`
(iii) directly as the sine of the angle (say `theta`) between `dvecl` and `hatr`
(iv) inversely as the square of the distance between the point P and length element `dvecl`.
This is expressed as
`d B prop (Idl)/(r^2)sin theta`