Photo above showing the wire frame where current will pass through. The frame is completely between the poles of the u-shaped magnet (North is red) during experiment.
Photo above shows that the wire frame is not in contact with the magnet.
Questions: What is the direction of the force on the wire frame and the magnet? Why is the magnet getting "heavier"? When the current is reversed (http://youtu.be/hnU4g4NBXXo), why is the magnet getting "lighter"?
F = BIL
Plot a graph of magnetic force F against current I in the wire frame. If a linear trend is obtained, the gradient will be BL where L is measured to be 4.0 cm.
Gradient = 4.466 mN/A = 0.004466 N/A
Therefore B = 0.004466/0.040 = 0.11 T
This value is reasonable because the magnetic flux density of the magnet should be much greater than Earth's intrinsic magnetic field (~10 nT) and flux density of more than 1 T is hard to obtain in common school lab.