Most materials exhibit a change of resistance with applied magnetic fields (magnetoresistance) that is one of the most common transport measurements made at NHMFL. Magnetoresistance measurements contain information about a material's electronic structure, carrier mass, electron-phonon coupling, etc. 4 wires need to be attached to a sample to measure it's magnetoresistance. Ordinarily, resistance is measured by placing two probes of an ohm-meter across the ends of whatever is being measured.
2 wire measurements do not work very well for samples located in our experimental set-up because the samples are located at the bottom of a 1.5 meter long probe. The leads connecting the sample to the outside world have a resistance of ~25W. This makes it impossible to measure accurately the resistance of just the sample using the 2 lead measurement method if the sample resistance is less than 25W. The 4 wire resistance measurement method solves this problem.
For the 4 wire method, we pass a current through the sample (2 contacts), then measure the potential difference across the sample (2 more contacts). We then know that R = V/I between the voltage leads, which gives us just the resistance of the sample without the leads and contacts.