======Measuring the Gravitational Constant======


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**Alternative titles:** The Cavendish Experiment
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====Summary====                   
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====Procedure====  
  - Suspend a lightweight rod horizontally using a fine torsion wire.  
  - Attach small lead spheres to each end of the rod.  
  - Place two much larger lead spheres close to the smaller ones, so that their gravitational attraction slightly twists the torsion wire.  
  - Observe and record the tiny angular deflection of the rod using a scale or mirror and light beam.  
  - Repeat the experiment by moving the large masses to opposite sides and measuring the change in deflection.  
  - Calculate the gravitational constant (G) using the measured forces, masses, and distances.



====Links==== 
The Cavendish Experiment - Obvious Gravitational Attraction - MrLundScience: 
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Watch gravity pull two metal balls together - Steve Mould: 
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====Variations====
  * 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).



====Safety Precautions====
  * 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.




====Questions to Consider====
  * 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.)