A long, straight, hollow conductor (tube) carrying a current has two sections A and C of unequal cross-section joined by a conical section B. 1, 2 and 3 are points on a line parallel to the axis of the conductor. The magnetic fields at 1, 2 and 3 have magnitudes B₁ , B₂ and B₃ . Then

• A

  B₁ = B₂ = B₃

• B

  ${\mathrm{B}}_1={\mathrm{B}}_2\ne {\mathrm{B}}_3$

• C

  ${\mathrm{B}}_1<{\mathrm{B}}_2<{\mathrm{B}}_3$

• D

  B₂ cannot be found unless the dimensions of the section B are known