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1L10.30 - Cavendish Experiment

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​1L10.30 - Cavendish Experiment

Title1L10.30 - Cavendish Experiment

The Cavendish experiment was designed to measure G, the gravitational constant.​ **24 Hour notice required**

Assembly Instructions

**A 24 Hour Notice Required to use this demonstration**


Set up the Cavendish apparatus and laser. Allow the apparatus to stabilize for 10-15 minutes while using a hand magnet to stop oscillations of the internal masses which are diamagnetic (lead). The large external masses must initially be perpendicular to the plane of the internal masses. After the beam spot has stabilized on the wall, mark the spot on the wall with a piece of masking tape. After measuring the wall to Cavendish distance, carefully move the external masses to one extreme and let the beam restabilize for 15-20 minutes. Measure the displacement between the tape and the beam. Unfortunately, the torsion constant of the string is unknown, so a quantitative measurement cannot be made.
Small Mass:
m = 0.015 kg
r = 6.9 mm
Large Mass:
m = 1.5 kg
r = 32 mm
Torsion Bar: R = 50 mm
Torque Applied on the order of 10^-9 N

Setup Time20
Operation Time
Preview Time5
Operation Instructions

**A 24 Hour Notice Required to use this demonstration**

Demo on DimeNo
PIRA 200Yes
Export Instructions (if different)
HazardsFragile: Caution when Transporting, Light Hazard - Laser Radiation Class IIIB

Cavendish Lab, in Cambridge University, is the laboratory with the most Physics Noble Prize winners. This is in great part by the precedence that Henry Cavendish as a brilliant experimenter left to the laboratory. His most famous experiment was aimed at measuring Newton’s constant “G”, which appears in the universal gravitational force law. Since mass is positive, gravity is always an attractive force. Unfortunately, Newton’s constant is incredibly small, which makes the attraction between two masses almost impossible to measure in a laboratory setup. Cavendish was the exception, he was able to design an experiment that would measure the most sensitive of attraction between two masses by tying them to a beam in the center and measuring the torsion produced by the gravitational attraction. The number of windings that the two masses did around the center beam, is directly correlated to the strength of the gravitational force between them. This way, Cavendish was able to measure Newton’s Constant within 1% of today’s accepted value.

Category1 Mechanics
Subcategory1L - Gravity
Construction Information
Cavendish apparatus
laser - green 4-mW
meter stick
magnet - Alnico cylindrical 3"