Measuring the Gravitational Constant

The Cavendish Experiment, first performed in 1797–98 by Henry Cavendish, measures the tiny gravitational attraction between lead spheres using a torsion balance. From this, the gravitational constant (G) can be determined.

1. Suspend a lightweight rod horizontally using a fine torsion wire.
2. Attach small lead spheres to each end of the rod.
3. Place two much larger lead spheres close to the smaller ones, so that their gravitational attraction slightly twists the torsion wire.
4. Observe and record the tiny angular deflection of the rod using a scale or mirror and light beam.
5. Repeat the experiment by moving the large masses to opposite sides and measuring the change in deflection.
6. Calculate the gravitational constant (G) using the measured forces, masses, and distances.

The Cavendish Experiment - Obvious Gravitational Attraction - MrLundScience:

Watch gravity pull two metal balls together - Steve Mould:

• Use different torsion wire thicknesses to compare sensitivity.
• Try different materials or sizes of test masses to observe how gravitational force depends on mass.
• Demonstrate a simplified version with lightweight spheres and visible deflection (though not sensitive enough to measure G).

• Ensure the torsion balance setup is stable and isolated from vibrations and air currents.
• Handle heavy lead spheres carefully to avoid injury.
• Avoid direct contact with lead; wash hands after handling or use gloves.

• Why was the Cavendish Experiment historically important? (It provided the first accurate measurement of G and allowed calculation of the Earth's mass.)
• Why must the experiment be isolated from vibrations and air currents? (Because the gravitational force measured is extremely small and easily disrupted.)
• How does the gravitational force change with distance between the masses? (It decreases with the square of the distance, according to Newton’s law of universal gravitation.)