The magnetic flux in the region between the poles of a magnet can be measured using the apparatus below. The magnetic field between the poles of the magnetic is approximately uniform.
The length L of the wire between the magnets can be measured with a ruler. When no current flows, the weight of the magnet can be read off the top pan balance. When a current flows as shown above, the wire experiences a force, the direction of which is given by Fleming's Left Hand Rule and the magnitude of which is given by the equation
where
= magnetic field strength between the magnet poles.
= current
= length of wire between the magnets.
Fleming's Left Hand Rule gives the direction of the force on the wire as upwards, so the wire will exert an equal force downwards on the magnet, increasing the reading on the top pan balance. If 
and
are the readings on the top pan balance before and while a current flows respectively, then