One of the greatest advantages of superconductors is their ability to carry large currents through small areas. A major problem however, is getting the current in and out. Consider a large water main for a major metropolitan city. If the ends of the pipe were small and difficult to attach, the limiting factor would be the connecting pipes not the main. This can be a significant problem with superconductors. They may have very high current densities, but if there is too much resistance in the electrical contacts the current flow will cause heating at the contacts. Power dissipation as heat is equal to the current flow multiplied by the voltage drop. Since superconductivity exists only at low temperatures, this heating causes easy flow of electrical current to be limited. Attaching contact leads to bulk superconductors can be difficult. Many commercially produced superconductors can by purchased with electrical leads already attached. The contact resistance of commercial superconductors are usually superior to contacts that are made in a high school lab.
One method for attaching contact leads to your superconductor requires small gauge silver or copper wire and electrical silver paint. These items are readily available at any electronic supply store. By attaching the leads to the sample with the silver paint as shown in Figure (17) you should have a good four point probe. If the contact resistance is too high, scrape the silver paint away and then try resintering the sample by annealing it again in your oven. When a simple measurement of electrical resistance is made by a two probe connection, you unintentionally also measure the resistance of the contact points. In a normal conductor the resistance of the contact points is relatively small compared to the resistance of the conductor. However, when measuring the resistance of a superconductor, which has very low or no resistance, the contact resistance is very significant.
The effect of the contact resistance can be diminished by the use of a
four point probe. Figure (17) shows a four point probe attached to a
superconducting sample. A constant current flows through the wires labeled
C and C
. A power supply capable of providing 0.5 amperes
is needed. If
an ammeter is not built into the power supply an ammeter should be wired
into the circuit to monitor the current flow.
If there is any resistance in the sample, a voltage drop will occur
between the contact points of current leads as the current spreads and flows.
Wires V and V
are connected to a digital volt meter to detect
any voltage
drop between their contact points. If these two contacts are physically
removed from the current contacts (wires labeled C
and C
),
the resistance of
the superconductor between wires V
and V
is the ratio of
the voltage to the
output current of the power supply, according to Ohm's Law V=IR. If wires
V
and V
contact wires C
and
C
, the measured resistance R is the sum of
the sample resistance and the two contact resistances.
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