Hall Effect
In the Hall effect experiment you take a strip of metal that is running a current and put it in a magnetic field. The negative charges in the current will be pulled towards one side of the metal strip, the side closest to the magnetic field, and the positive charges will be pushed to the other side. This creates a voltage across the current.
After the magnetic field is applied we use some electronic equipment to measure the density of the charge carriers (called, fortuitously, the Hall coefficient). Sound simple, well, I'm cutting the explanation short to keep it simple. Believe me the electronics involved make this a non-trivial experiment.
The picture below is donated by Joshua Santarpia and is a plot of the hall voltage vs. the applied B field. The charge density is obtained from the slope of the graph.