Franklin’s Bells

A small conductive ball swings back and forth between two metal plates, alternately charging and discharging as it contacts each plate.

1. Suspend a small metal ball on a string so that it hangs freely between two unconnected metal plates.
2. Ground one plate by touching it with your finger.
3. Rub a plastic rod (e.g., PVC or acrylic) with wool or fur to give it a static charge.
4. Bring the charged rod near one of the plates so that it charges up by induction.
5. The ball is attracted to the charged plate, touches it, and becomes charged.
6. Once charged, the ball is repelled and swings toward the grounded plate.
7. After contact, the ball picks up the opposite charge and is pulled back toward the charged plate.
8. The ball continues to swing back and forth, transferring charge between the plates.

DEMO: Franklin's Bell - Professor Hafner:

Franklin's Bell - How it Works - RimstarOrg:

• Try using different materials (PVC, acrylic, hard rubber) for the rod and compare effectiveness.
• Use two differently charged rods to test how opposite charges affect the behavior.
• Perform the demo in a darkened room to see small sparks at contact points.

• Do not let students touch the plates while the rod is charged.
• Ensure the suspended ball is securely attached so it does not fly off.
• Keep the charged rod away from sensitive electronics, as static discharges can cause damage.

• How does the plastic rod charge the plate without touching it? (By induction, it repels or attracts electrons in the plate.)
• Why does the ball keep moving back and forth instead of stopping after one swing? (It alternately picks up charge from each plate, causing continual attraction and repulsion.)
• How does this version compare to using a Van de Graaff generator? (The principle is the same, but the charge from a rod is weaker and may not sustain oscillations as long.)
• What natural phenomenon was Franklin trying to detect with his lightning bells? (The presence of atmospheric electricity during thunderstorms.)