Equipment: An electromagnet, a variac, a set of rings (aluminum rings of varios diameters, one aluminum ring with a slit and one copper ring), forceps, a small (~2L) dewar of liquid nitrogen, safety goggles and safety gloves.

What it does: The elctromagnet levitates the solid aluminum rings and causes them to 'jump' when the power is turned on suddenly. The physical property of the ring determines if and how high it will jump. The ring with the slit and the copper ring will not jump. A ring cooled in liquid nitrogen will jump higher than an identical ring that is at room temperature.

Concepts Demonstrated: Magnetic levitation. The relationship between levitation and temperature, material and other physical properties of an object.

Classroom assembly:

1. The set of rings contains two identical aluminum rings. Place one in the dewar of liquid nitrogen to cool.
2. Raise the electromagnet core so that it is about 75% above the frame and secure it in place using the set screw.
3. Plug the electromagnet into the variac.
4. Turn the variac to about 100V.
5. Place a ring on the electromagnet.
6. Turn on the variac. The ring will jump off the electromagnet. Catch it.
7. Turn off the variac.
8. Repeat for a variety of rings including the identical rings that are at different temperatures.

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

Setup Time: Short.

Difficulty: Straightforward.

Visibility: High. This demonstration is appropriate for large lecture halls.

Related demonstrations:

References: PIRA 5K20.30

Original Construction: The set of rings were cut from stock. All other equipment was purchased.