The apparatus shown in the figure below is used in an experiment to find the magnetic flux density 
between the poles of a horseshoe magnet. Assume the magnetic field is uniform between the poles of the magnet and zero elsewhere.
The rigid metal rod of length 
is fixed in position perpendicular to the direction of the magnetic field. The poles of the magnet are both 
. There is a current in the rod that causes a force on the rod. The balance is used to determine the magnitude of the force. The variation with current 
of the force 
on the rod is shown in the graph below.
Calculate the magnetic flux density .
.
