**EXPERIMENT #2**

**Resistance/Temperature Relationship**

*Objective:*

- To obtain a resistivity vs. temperature curve:
- 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:*

- Attach current leads, (C1,C2) to the power supply.
- Attach millivolt meter to leads V and V.
- Attach thermocouple to millivolt meter and calibrate.
- Carefully immerse the superconductor into liquid nitrogen.
- When the boiling stops carefully adjust the current to 0.5 amps.
- 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).
- Allow the liquid nitrogen to slowly boil away permitting the
superconductor to slowly warm.
- * 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)*