Learning Library

# Electromagnetic Ring Launcher

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### Problem

What material will a ring launcher toss highest?

### Materials

• Elihu Thomson apparatus (aka ring launcher)
• Copper ring
• Aluminum ring
• Split aluminum ring
• Copper collar
• Freezer
• Liquid nitrogen (optional)
• Tongs (needed if you use liquid nitrogen)

Safety: Wear eye goggles. Make sure no one and nothing will be hit by flying rings. Take caution when dealing with the ring launcher and rings, as they may get hot. Also be sure to be careful when dealing with freezing rings.

### Procedure

1. Pick a ring to test.
2. Slide the ring onto the ring launcher over the coil.
3. Apply current to the ring by switching the launcher on.

2. Freeze the rings overnight. Repeat steps 1-4. Be sure the ceilings have high clearances. Better yet, get an extension cord and perform this part of the experiment outside.
3. Using the tongs, submerge the rings in liquid nitrogen.
4. Using the tongs to handle the rings, repeat steps 1-4. Do not use your bare hands to handle rings frozen with liquid nitrogen.

### Results

The copper ring will be tossed the highest, followed by the aluminum ring. The copper collar will lift slightly, but will not be launched. The split rings will not move.

Frozen rings will be thrown much farther than room-temperature rings.

### Why?

Closed loops that carry current produce their own magnetic fields. The alternating (direction-changing) electric current from the power outlet in the wall charges the core, which then creates a magnetic field. The magnetic field then induces a current in the closed rings, which then create their own magnetic fields. When the magnet fields repel each other, the ring is launched. The split rings do not carry enough current to and do not create a magnetic field that is equally distributed around the ring.

Cooper launches the highest because it is the best conductor of electricity. The larger the change in the magnetic flux, the greater is the force that tosses the ring. Aluminum is the next best conductor, followed by lead.

Similarly, frozen rings experience a very large change in magnetic flux when they are suddenly heated with induced current, causing them to fly much higher than they typically do at room temperature.

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