Copper has positive temperature coefficient; light bulb gets brighter when copper leads are dipped in liquid N2.
What it shows:
Copper has a positive temperature coefficient (≈ 3.9×10-3 per ˚C), which means that its resistance drops with temperature. Here copper wire is immersed in liquid nitrogen (77˚K = -196˚C), decreasing its resistance (from room temperature) by almost a factor of 2, thus increasing the current flow though a circuit.
How it works:
We have a coil of 30AWG copper wire wrapped around a (10×3cm) plastic spool, connected in series with a 40W 110V light bulb. The separation of bulb and coil is about 50cm - enough that the coil can be lowered into a dewar with the bulb sitting on the bench. The coil's resistance at room temperature is 30Ω, which lets the bulb light dimly at 300K. As the coil cools to liquid nitrogen temperature, the bulb gets rapidly brighter.
Setting it up:
Use a clear 4L dewar a third full of LN2.
A good companion demo to Carbon Resistor & Bulb, which shows the opposite effect on carbon.