A perfectly conducting straight rigid rod of mass m and length L is suspended symmetrically from two identical perfectly conducting springs as shown in the figure. The springs stretch a distance of due to the weight of the wire. The circuit has a total
resistance of . When the magnetic field perpendicular to the plane of the paper is switched on, springs are observed to extend further by the same distance. The magnetic field strength is
directed outward from the plane of the paper
; directed outward from the plane of the paper
; directed into the plane of the paper
; directed into the plane of the paper