Lab 8: Resistance as a Function of Temperature
Lab 8: Resistance as a Function of Temperature
In this lab, you are going to measure the resistance as a function of temperature of two samples – a long copper wire and a long nichrome wire. The samples are wound around a stem which can be inserted directly into a nitrogen dewar or thermal mug. (See the pictures below.)
You are going to measure the resistance of the samples at four or five temperatures. Measuring at room temperature is easy. You can also obtain thermal baths of hot (recently boiled) water, and ice water. You may be able to do one intermediate temperature (by waiting for the water to cool or adding a bit of cold water to a hot sample). You will use the liquid nitrogen dewar to do the coldest temperature of the set. Liquid nitrogen boils at 77 K, you will use this as the temperature; DO NOT put a thermometer into the liquid nitrogen dewar.
We suggest that you measure the room temperature resistance of both wires first. (Once they’ve been heated or cooled, you will have to wait for them to return to room temperature.)
Experimental Setup:
How can you determine an unknown resistance? Use a bridge circuit!
Setup a bridge circuit using the circuit board and potentiometer as we did in the DC3 lab. This time your second branch will contain a 10Ω resistor and the unknown copper or nichrome wire.
Since it will be your first measurement, you may as well use the room temperature resistance to test your bridge circuit. Remember to measure and record the temperature of the room! Use an ohmmeter to measure the resistance of the two wires directly and then make sure that your bridge measurement returns these values.
At each temperature, you will want to measure the resistance three times. (You never do a measurement just once, right?) We suggest that you alternate between measurements of the copper and nichrome wires (i.e measure copper, then nichrome, then copper, then nichrome, etc.) This will force you to reset the bridge every time and ensure three independent measurements.
You will need to wait for your sample to come to thermal equilibrium. When the temperature settles, the resistance will settle. Remember to check your thermometer at the beginning and end of each temperature run in the water baths – the temperature may change a bit during the measurement.
Every measurement has an uncertainty which must be assessed at the time the measurement is made. What is the uncertainty on the potentiometer dial reading? Read your meters carefully: is the measurement 10? 10.0? 10.00? The uncertainty is much smaller on the last possibility than on the first!