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
- superconductor with attached leads
- AC power supply
- liquid nitrogen
- dewar or styrofoam container
- 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.
- 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.
- Set the oscilloscope trigger to display one sweep across the screen
for each ac cycle.
- Adjust the input voltage sensitivity setting on the oscilloscope as
needed to obtain an image similar to Figure (22).
- 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.
- Remove the superconductor from the liquid nitrogen. The resistance
increases causing the voltage drop to increase again.
- 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
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
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
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