Equipment: A flashlight connected in series with a wire coil, a small (~2L) dewar of liquid nitrogen, safety goggles and safety gloves.

What it does: When the coil is at room temperature, the electrical resistance of the coil is high and the flashlight does not light up. When the coil is cooled in liquid nitrogen, the resistance drops and the flashlight bulb glows.

Concepts Demonstrated: The temperature dependence of resistance.

Classroom assembly:

1. Turn on the flashlight. Point out that the flashlight does not light up.
2. Dip the coil in liquid nitrogen. WEAR APPROPRIATE SAFETY GEAR. Point out that the flashlight lights up and grows brighter as the temperature of the coil decreases.

• The bulb may glow dimly at room temperature if the batteries in the flashlight are new.
• The coil warms quickly when removed from the liquid nitrogen. the bulb brightness decreases rapidly as the coil warms.

Cautions: Liquid nitrogen can cause severe burns and blindness. Use with caution.

Setup Time: Short.

Difficulty: Straightforward.

Visibility: High. This demonstration is highly visible when the lights are dimmed and is appropriate for large lecture halls.

Related demonstrations:

References: PIRA 5D20.10
Video encyclopedia 17-21

Original Construction: A standard flashlight was modified by soldering the flashlight bulb in series with a coil of wire. The coil consists of many turns of 24 gague copper wire. When constructing, add turns to the coil until the flashlight does not light up at room temperature