This experiment measures the Critical Magnetic Field (Hc2) of a ceramic superconductor using the four point probe device. Equipment to measure Hc1, the lower Critical Field is beyond the scope of our approach.
For this experiment you will need an electromagnetic coil and associated power supply. The value of the field can be obtained using the geometry of the coil and a knowledge of the current flowing through it. The cavity in the middle of the coil needs to be large enough to accommodate the four point probe device and the liquid nitrogen container in which it is immersed. The four point probe device has been designed without any ferromagnetic parts to eliminate any potential interference.
Assemble the experiment as was done in preparation of the measurement of Critical Current Density, however, this time place the four point device and its container of liquid nitrogen inside the cavity of the electromagnet. Gradually increase the current flowing through the electromagnet thus increasing the magnetic field strength through the superconductor. The value of V23 will show an abrupt increase at some value of applied magnetic field strength. This value of magnetic field is the upper Critical Magnetic Field, Hc2 for the superconductor sample at the temperature of liquid nitrogen, 77 Kelvin.
The value of the Critical Field, Hc2 can be obtained at other temperatures by either placing the device in a cryostat while performing this experiment, or by removing the device from the liquid nitrogen container and monitoring the output of the thermocouple thermometer while measuring Hc2.
Another interesting experiment is the measurement of the Critical Temperature at different applied magnetic field strengths. The result of such an experiment for YBa2Cu3O7 is shown schematically in figure 6. The value of Hc2 has been extrapolated to 0 Kelvin. It is very instructive to perform a subset of this experiment using the square neodymium magnet provided with your kit instead of an electromagnet. As the magnet is brought close to the surface of the superconductor device the value of V23 will slowly increase. This phenomena could potentially be used to construct a superconductor-based magnetic field detector.