### EXPERIMENT #1

#### Determining T using the Meissner Effect:

One very simple method for measuring the critical temperature of a superconductor is by using the Meissner Effect. Remember when the temperature of a superconductor is lowered to below the critical temperature, (Tc), the superconductor will "push" the field out of itself. It does this by creating surface currents in itself which produces a magnetic field exactly countering the external field, producing a "magnetic mirror". The superconductor becomes perfectly diamagnetic, canceling all magnetic flux in its interior. This perfect diamagnetic property of superconductors is perhaps the most fundamental macroscopic property of a superconductor. When your material is superconducting the magnet will begin to float above the superconductor. If you monitor the temperature as the Meissner effect occurs you can obtain a good approximation of the critical temperature.

Objective:

1. To determine the approximate value of a YBCO superconductor using the Meissner Effect.

Materials:

• YBCO superconductor with attached thermocouple

• small magnet

• millivolt meter

• liquid nitrogen

Procedure:

1. Attach the thermocouple lead from the superconductor to a digital voltmeter. The voltmeter should be set to the millivolt range.
2. Completely immerse the superconducting disk in liquid nitrogen with the thermocouple on the bottom. Calibrate your thermocouple according to the manufacturer's specifications.
3. Carefully balance your magnet above the superconducting material and observe the Meissner Effect. When the liquid nitrogen has almost completely boiled away the temperature will begin to increase.
4. Observe the magnet as the disk warms. The magnet will stay levitated as long as the disc is superconducting. As the disk warms the magnet will slowly begin to fall. Record the temperature when the magnet comes to a complete rest on the superconducting disc.

Remember the disk will not be warming equally throughout. The top of the superconductor will warm up before the bottom of the disk which is still in liquid nitrogen. Because YBCO disks are bulk superconductors, the Meissner effect will diminish as the top of the disk warms until the entire superconductor is above the critical temperature.

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Date posted 04/01/96 (ktb)