### EXPERIMENT #2

#### Resistance/Temperature Relationship

Objective:

1. To obtain a resistivity vs. temperature curve:
2. To determine the critical temperature of a YBCO superconductor;

Materials:

• YBCO superconductor with four point probe and thermocouple

• millivolt meter

• constant current supply

• ammeter

• liquid nitrogen

Procedure:

1. Attach current leads, (C1,C2) to the power supply.
2. Attach millivolt meter to leads V and V.
3. Attach thermocouple to millivolt meter and calibrate.
4. Carefully immerse the superconductor into liquid nitrogen.
5. When the boiling stops carefully adjust the current to 0.5 amps.
6. Record the temperature and the voltage across V and V. This voltage is expected to be the same with or without current flow because it is the thermal electromotive force (EMF).
7. Allow the liquid nitrogen to slowly boil away permitting the superconductor to slowly warm.
8. * Record the voltage drop and temperature as the superconductor warms.The electrical power supply should be turned off and/or disconnected periodically and thermal EMF recorded.

By knowing the voltage drop across leads V and V, and the current flowing through the circuit the resistance of the superconductor can be calculated according to Ohm's Law.

• Make a graph plotting the resistance in ohms on the Y axis and the Kelvin temperature on the X axis.
• Determine the critical temperature for your superconductor.

• Repeat this experiment using 0.2 amps.

• Calculate the resistivity of your sample. (ohms-cm)

* Note: Results from this experiment can be greatly improved if the temperature and voltage drop values can be recorded by a strip chart or by interfacing with a computer.

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Date posted 04/01/96 (ktb)