Current Balance Demonstration/Experiment

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. 

The video of the experiment is embedded below or http://youtu.be/nkS7wCCzGiY. You may pause the video at each current reading to obtain the following data. The balance is zeroed to measure ony the gain/loss in mass of 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"?

The electronic balance gives reading in grammes. The magnetic force is obtained by multiplying the balance reading by 10 N/kg (or 9.8 N/kg if you want to be accurate). Since

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.