The magnitude of dH at point p is proportional to:

• Product of current, differential length dl, since of angle between wire and line connecting point P.
• Inversely proportional to the square of distance

• Constant 4π

  Mathematically

  $dH=\dfrac{I dl. \sin⁡θ}{4πR^2}$

  In vector form

  $d \bar{H}=\dfrac{I dl × \overline{a_R}}{4πR^2} (A/m)$