### EXPERIMENT #4

#### Determination of the Critical Magnetic Field

Objective:

To determine the critical field, H  , of a YBCO superconductor.

Materials:

• YBCO superconductor with attached leads

• power supply

• electromagnetic coil

• millivoltmeter

• liquid nitrogen

• dewar or styrofoam container

Procedure:

1. Connect current probes and voltage probes as done for Experiment #3. For study of a disk superconductor adjust the current to 0.5 ampere (or 0.05 ampere for coil).
2. Connect power supply to electromagnetic coil and place the coil into liquid nitrogen. The value of the magnetic field can be determined by knowing the geometry of the coil and the current passing through it. The coil must be large enough to place the superconductor in it.

3. Place the superconductor into the coil in the liquid nitrogen. Record the voltage drop (V , V ) and the magnetic field applied. This field strength must be determined by multiplying the current supplied by a calibration constant for the coil used. An approximate expression is: where the solenoidal coil of diameter d has NL turns over its length L. The magnetic field H is in units of Oersteds and lengths are in centimeters.

4. Gradually increase the magnetic field placed on the superconductor. Record the voltage and magnetic field for each setting.
5. The voltage (V , V ) will show an abrupt increase at some value of the applied magnetic field strength. This value represents the critical magnetic field H  , for the superconductor at 77 K.

• Make a graph plotting applied magnetic field strength on the Y axis and resistance on the X axis.

• Determine the critical magnetic field strength for the superconductor at 77 K.

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Date posted 04/01/96 (ktb)