DEMONSTRATION #3

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Voltage Drop using an Oscilloscope

The voltage drop that occurs when a superconductor goes from the normal state to the superconducting state can be easily demonstrated on an oscilloscope.The nonlinear current-voltage relation of a superconductor in its mixed state may also be observable if a hand-held magnet can apply enough magnetic field.

Materials:

• Oscilloscope

• superconductor with attached leads

• AC power supply

• liquid nitrogen

• dewar or styrofoam container

Procedure:

1. Connect the power supply, superconductor, and oscilloscope as shown in Figure (21). Figure (23) shows a circuit diagram for an ac ramp power supply made from readily available components.
2. The voltage drop across the voltage leads (V, V) will be seen on the "Y" axis if connected to the vertical input terminals. Note that the ac power supply floats with respect to the ground at the oscilloscope input. If you wish to use a commercial supply with a grounded output, use a differen- tial input amplifier available on many oscilloscopes. Figure 24 shows the circuit diagram for a differential amplifier that can be built from readily available components.

3. Set the oscilloscope trigger to display one sweep across the screen for each ac cycle.
4. Adjust the input voltage sensitivity setting on the oscilloscope as needed to obtain an image similar to Figure (22).
5. Place the superconducting disk into the liquid nitrogen. As the disk cools and begins to superconduct the voltage drop across the voltage leads (V, V), will go to zero. This will cause the image to rotate to a horizontal line.
6. Remove the superconductor from the liquid nitrogen. The resistance increases causing the voltage drop to increase again.
7. Place the superconductor back into the liquid nitrogen and watch the voltage drop back to zero. Now place a strong magnet near the superconductor and observe the voltage drop. Under the right conditions, resistivity will again appear. The supercurrent vortices move (indicated by the observed voltage that is not quite proportional to electrical current) in reponse to a driving force due to the current flow.

Figure (23) shows a simple circuit diagram for an ac ramp power supply. It can be built with inexpensive and readily available 741 operational amplifiers, a 2N6058 transistor, two 1N4001 diodes, two 0.1 F capacitors, two ordinary 9-V batteries,and various resistors. This ramping current supplies current that increases linearly with time up to an adjustable maximum value (amplitude). Then it drops to zero and repeats the ramp at an adjustable frequency.

Figure (24) is a schematic diagram of a simple differential amplifier. Connecting the superconductor directly to grounded oscilloscope input gives a terribly inaccurate voltage measurement for the oscilloscope's vertical deflection, the Y value of the displayed graph. Voltage connections V and V give approximately the same voltages at both inputs to the amplifier. These voltages must be rejected except for their small difference, the voltage drop within the superconductor. An ordinary oscilloscope with two inputs that can be subtracted for display will show graphical values of the difference that includes "common mode" error. The gains for the inputs that are electronically subtracted are R/R and R/R.

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