PURPOSE: To demonstrate the relation between resistivity and temperature

DESCRIPTION: A coil of common iron baling wire (wound on a hollow Pyrex glass cylinder) is wired in series with a 24-volt transformer and a 10-volt galvanometer lamp (code GE 3.5A/T14). A Variac adjusts the voltage coming from the 24V transformer until the lamp burns almost at normal brilliance. The coil of iron wire is then heated to red heat with a torch; the lamp dims and goes out. When the torch is removed and the coil cools, the lamp gradually resumes its original brightness. For maximum effect, this should be done in a semi-darkened room.

SETUP TIME: Moderate Difficulty

COMPONENT LIST:

  Iron wire/Pyrex cylinder			Emag Shelves

  24-V transformer				Power Supplies

  10V galvanometer lamp			Lamps

  Variac					Emag shelves (bottom)

SUGGESTIONS:

REFERENCES: 2122 6.5;PIRA 5D20.20

OTHER NOTES: Iron is chosen because of its high temperature coefficient of resistivity over the range of temperature involved here. Specifically, if iron is assigned a resistivity of 1.0 (arbitrary units) at 0^o C, at 1000^o C the resistivity will be 13.07; for comparison, copper is about 6 and nickel is about 7.5 (source: AIP Handbook, 3rd edition, Table 4a-13). As a side note, the resistivity of every metallic element increases with increasing temperature, with the sole exception of plutonium (K&L p. 118), a material whose use in elementary demonstrations is frowned upon, especially since it catches fire at red heat, evolving copious radioactive smoke.