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 the points on a line parallel to the axis of the conductor. The magnetic fields at 1, 2 and 3 have magnitude B₁, B₂ and B₃ :

• A

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

• B

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

• C

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

• D

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