A Cavendish torsion balance demonstrates the force of gravity between two masses and allows the gravitational constant to be determined. Thanks to the short oscillation period of just 2–4 minutes, the gravitational constant can be determined within the space of a single lesson with an accuracy of better than 10%.
The core of the apparatus is a torsion pendulum made of a light bar with two small lead spheres, which is suspended horizontally from a thin wire. The apparatus is moved from its rest position by the attraction of the two spheres to two larger lead spheres. When the two large spheres are rotated to a new position, the torsion balance will vibrate about a new rest position. The rotary motion is measured using a capacitive differential sensor, which largely suppresses noise and vibration components in the signal. The output is then recorded using a computer. For subsequent evaluation, the data can be exported to a spreadsheet. Alternatively, the motion can be demonstrated with the aid of a light pointer.
Mass of large lead spheres: 1 kg
Mass of small lead spheres: 15 g
Gravitational attraction: < 10–9 N
Torsion wire: Tungsten, 25 µm
Period of oscillation: 2–4 mins
Angular resolution: 25 microradians
Sampling rate: 0.5, 1, 2, 5, 10 samples/s
Dimensions: 190 x 180 x 200 mm³
Weight: 5 kg
1 Cavendish torsion balance
1 Measurement software
1 USB cable