Resistance ** 5D20.10 ** Resistivity and temperature

Wire coil in liquid nitrogen

WARNING: The suicide coil used to supply power from the wall socket to the circuit is exposed and dangerous. Make sure it is switched off before touching the leads.

Equipment :

• wire coil wrapped around pencil - shelf 11E
• suicide 120V AC cord - shelf 11C
• light-bulb socket - shelf 11C
• 200 W bulb - shelf 12C
• PASCO digital meter - shelf 14D
• two banana-to-alligator leads - rack on door
• one alligator-to-alligator lead - rack on door
• 500 ml Erlenmeyer flask - over sink
• funnel - shelf 7E
• dewar for liquid nitrogen - shelf 6B
• liquid nitrogen

Theory: The resistivity of most materials is temperature-dependent. Copper has a resistivity which decreases with temperature; the resistance is about 2 times greater at room temperature (293 K) than it is at liquid nitrogen temperature (77 K).

Description: Begin by measuring the resistance of the coil at room temperature; it should be about 20 ohms. Then set up a circuit with the 200 W bulb in series with the coil, powered by 120 V AC. Don't leave this circuit on too long, or the coil will get too hot! In this configuration, the bulb should glow dimly. Place the coil in the flask, and pour in liquid nitrogen via the funnel. As the coil cools and its resistance drops, the light bulb will glow brightly. Measure the coil resistance, which should be about 8 ohms.

Setup time: Requires 24 hours notice unless the instructor supplies the liquid nitrogen; 15 minutes in that case.

References: (available in the demo room)

• Sutton, "Demonstration Experiments in Physics", H-104, p. 226.
• Rueckner and Steel, "The Demonstrative Physicist's Companion", TP5.10.