# The Low Temperature Probe

## The Complete Apparatus:

I have used the above equipment to make DC resistance measurements of both Copper and Superconductor samples in the High School classes. The dewar contains Liquid Nitrogen which has a boiling point of 77 Kelvin (or if you prefer about -195 Celcius). The samples are attached to the probe which is suspended within the narrow quartz tube. Since the probe is not in direct contact with the Liquid Nitrogen, it takes around 10 minutes for the sample to cool to 80 Kelvin. The shaft of the probe is made of stainless steel tubing because its low thermal conductivity minimizes the amount of heat conducting down from above.

The small grey box contains some amplifier circuitry used to aid in the resistance measurement described below. A simple diode attached to the probe is used to determine its temperature. The ribbon cable leaving the grey box is connected to a PC via A/D cards so that the results can be viewed graphically in real time.

Both samples have room temperature resistances that are less that 10 micro ohms which means that it is impossible to measure their resistance with a simple multimeter. To avoid the problem of measuring the resistance of the leads to the sample, I use a "4-wire" test. To do this, separate wires are used for sending current through the sample and for measuring the accompanying voltage drop. The voltage drop across the sample is never more than 100 millionths of a volt and thus requires a rather substantial amplification before the computer can accurately measure it. The resistance of the diode, which varies linearly with temperature in this temperature range, is also measured with a 4 wire method but its voltage is large enough to be measured without any amplification.

## The base of the probe:

The above photo shows the base of the probe with the Superconducting sample attached. The sample is the round black disk beneath the small red clip. The 4 wire electrical conection is accomplished with the red clip. If you look closely you can see the small Gold (bright yellow) wires that are used to make the final connection to the sample.

## The Two Samples:

Here you can see both samples side by side. They are attached to the end of the probe by a thin layer of grease. The copper sample is actually only a 1 cm length of #22 Cu wire that is glued down to the disk.

## The DC Amplifier:

The grey amplifier box contains the above circuitry. Notice that the sample has 4 separate wires attached and that the outer two are used to provide the current while the inner two monitor the voltage drop. The gain on the amplifier is in the order of 50 000. Notice that the diode current is quite small at only 30 microamps.

## Links To and Fro..

• Back to the Physics Outreach Home Page
• Back to the Superconductor Demo Menu
• Onward to Measurements on Copper