Sixth Experiment: Four-Point Probe
The four-point probe is a versatile instrument that can make a number of useful measurements. We will look at one popular experiment. Figure 6 is a schematic of a Four Point Probe; many times a thermocouple (not shown) is also included in the probe.
In this experiment we will pass 0.5 amps through the current probes (probe numbers 1 & 4). We will measure the voltage drop across the superconductor material from probes 2 and 3 using a digital voltmeter with a resolution of 0.01 mV.
Before the superconductor is chilled, you record the voltage reading (Voltage Drop) across voltage probes 2 and 3. Next chill the 4-point probe superconductor with liquid nitrogen. When the material cools down past its critical temperature it enters into a superconductor state. Now the voltage reading across probes 2 and 3 should drop to zero volts (voltage drop = zero).
If a thermocouple has been added to the 4-point probe, the temperate reading from the thermocouple at this electrical transition can also be recorded. This will be the critical transition temperature of the superconductor.
It should be noted that with these ceramic superconductors, the transition temperature is not a singular value, but usually a 5-degree range.