An experiment is conducted on a long straight wire of diameter d. A constant current is sent through the wire and the magnetic field on the surface of the wire is measured to beB₁. The experiment is then repeated with the same current but with a wire of diameter 2d. The magnetic field measured on the surface of this second wire will be which of the following?

B₁,  

B₁ / 4,  

4B₁,  

B₁ / 2