For another project that involved annealing transparent conductive oxides (TCO) on glass slides to observe a decrease in resistivity it was discovered that we had no way of measuring resistivity. We sought out a professor that we were told had a four-point probe and he mentored us on how one worked and how to make one.
The four-point probe was created using the sensor detached from a printer cartridge and connected, via solder and alligator clips, to a Keithley 2400 that could run a current (I) through the outside points of the probe and read a voltage (V) through the inside. Using Ohm’s law 𝑉/𝐼=𝑅 a resistance could be found and converted to a resistivity 𝑅=𝜌𝑑 /𝐴 where Area 𝐴=𝑑𝑡 d being distance between prongs and t being thickness of the film. This would result in resistivity being 𝜌=𝑅𝑡.