PURPOSE: To show zero resistance.
DESCRIPTION: A sample of Hi Tc material is mounted in a four
point probe device. The effects of contact resistance are eliminated. A schematic of a four point probe is shown in figure 2 below. In this
diagram, four wires (or probes) have been attached to the test sample. A power
supply is used to provide constant current that is made to flow the length of the sample through probes labeled
1 and 4 in the figure. This can be done using a current source or a power supply as shown.
The large display meter is used as a series ammeter to show the current output
to the class. A 5 Watt power supply capable of producing up to 0.5 Amp is
used.
The voltage drop between probes 2 and 3 can be measured and displayed by a digital
voltmeter. The resistance of the sample between probes 2 and 3 is the ratio of the voltage registering on the digital voltmeter to the value of the output
current of the power supply. The high impedance of the digital voltmeter minimizes the current flow through the portion of the circuit comprising the
voltmeter. Thus, since there is no potential drop across the contact resistance associated with probes 2 and 3, only the resistance associated with the
superconductor between probes 2 and 3 is measured.
EQUIPMENT: as photographed. This demo is sold as a kit by Colorado Superconductor.
SETUP NOTES: Adjust the voltage limit to max and set the current limit to 1/2 amp. If the voltage limit is not set to max then the current will drop when the superconductor is cooled because it will load the power supply and cause it to crowbar.
