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attachment:CavendishSpheres_5B3020.jpg
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The charge producers (rods and fur) are located in [:ElectrostaticsCabinet:Electrostatics Cabinet], A1. The 1000V DC Power Supply is located in [:ElectrostaticsCabinet:Electrostatics Cabinet], B2.
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Cavendish used this experiment to prove the inverse square law of electrostatic force. The proof plane is located in [:ElectrostaticsCabinet:Electrostatics Cabinet], A1.
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Connect the Faraday cage to the electrometer, grounding the outside cage. Connect two leads to the outside cage and simultaneously ground the inner and outer sphere. Cavendish used an apparatus consisting of two concentric spheres, insulated from each other and from ground, to prove the inverse square law of electrostatic force. This elegant experiment requires some investment of time by the demonstrator beforehand, to track down extraneous effects that could dominate the desired effect.
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Rub the black rod on the fur. The black rod charges negative, as can be demonstrated by bringing it inside the inside cage. Preliminaries:
 *Connect the Faraday cage to the electrometer, grounding the outside cage. Make sure the leads are twisted around each other, otherwise the electrometer measurements will be much more sensitive to charging due to the wires rubbing against each other if they happen to move slightly.
 *Connect a short lead (red wire in picture) to the outside cage, to be used to (momentarily) ground the inside cage in the event it accidentally gets charged.
 *Set the electrometer to 30V full-range, and to measure 0V at half-range.
 *Remove the top hemisphere.
 *Connect the bottom hemisphere to the outside of the cage (blue wire in picture).
 *Connect the positive 1000V terminal of the power supply to the inner sphere (yellow wire in picture).
 *Do not connect the ground terminal of the power supply to anything. Power supply and electrometer share a ground through the power cord.
 *Raise the inner sphere to 1000V potential.
 *Ground the proof plane, the proof plane handle, and your fingers by touching each of them to the outside cage.
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Touch the black rod to the lead to the inside sphere. The inside sphere is now charged negative, while the outside sphere remains neutral. At this point the bottom hemisphere is grounded and the inner sphere is at 1000V. Make some measurements to gain confidence:
 *Touch the proof plane to the inner sphere
 *Bring the proof plane inside the inside cage. Electrometer needle should deflect positive by about 6V. This indicates there is positive charge on the inner sphere
 *Ground the proof plane on the outside cage. Touch the proof plane to the outside of the bottom hemisphere. Bring the proof plane inside the inside cage. Needle should not deflect. This indicates there is no charge on the outside of the bottom hemisphere.
 *Ground the proof plane on the outside cage. Touch the proof plane to the inside of the bottom hemisphere. Bring the proof plane inside the inside cage. Needle should deflect negative almost 6V. This indicates there is negative charge on the inside of the outer hemisphere.
 *Disconnect the yellow wire from the inner sphere connector. Turn off the power supply.
 *Ground the proof plane on the outside cage. Touch the proof plane to the inner sphere. Bring the proof plane inside the inside cage. Needle should deflect, almost as much as before.
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Briefly connect the inside sphere to the outside sphere. Here is the actual Cavendish Experiment:
 *Replace the outer upper hemisphere.
 *Disconnect the grounding wire (ie the blue lead) from the outside cage, and touch it momentarily to the inner sphere connector. This brings the two spheres to the same potential.
 *Replace the grounding wire (the blue lead) on the outside cage. This grounds the outer sphere.
 *Remove the outer top hemisphere.
 *Ground the proof plane on the outside cage. Touch the proof plane to the inner sphere. Bring the proof plane inside the inside cage. The needle should not deflect!
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The inside sphere is found to be uncharged, all of the charge having migrated to the outside sphere. Discussion: The fact that the needle does not deflect on the last measurement shows that the inner sphere has been left neutral, and was proposed by Cavendish as a consequence of the inverse square law of the electrostatic force. If the electrostatic force dropped off faster than inverse square, the inner and outer spheres would end up with charges of the same sign. If the electrostatic force dropped off slower than inverse square, the inner and outer spheres would end up with charges of opposite sign. Only if the electrostatic force drops off as inverse square can the inner sphere end up with no charge.
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Note: this writeup is incomplete as of 5/30/08. No one at UW physics seems to have successfully used this demonstration. Cavendish's account of his experiment may be found in "The Electrical Researches of Henry Cavendish", edited by James Clerk Maxwell, paragraphs 217-234.
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attachment:CavendishSpheres5B3020.jpg

Cavendish Spheres 5B30.20

attachment:CavendishSpheres_5B3020.jpg

The Cavendish Spheres, Faraday Cage, and electrometer are located in [:ElectrostaticsCabinet:Electrostatics Cabinet], B1.

The 1000V DC Power Supply is located in [:ElectrostaticsCabinet:Electrostatics Cabinet], B2.

The proof plane is located in [:ElectrostaticsCabinet:Electrostatics Cabinet], A1.

Cavendish used an apparatus consisting of two concentric spheres, insulated from each other and from ground, to prove the inverse square law of electrostatic force. This elegant experiment requires some investment of time by the demonstrator beforehand, to track down extraneous effects that could dominate the desired effect.

Preliminaries:

  • Connect the Faraday cage to the electrometer, grounding the outside cage. Make sure the leads are twisted around each other, otherwise the electrometer measurements will be much more sensitive to charging due to the wires rubbing against each other if they happen to move slightly.
  • Connect a short lead (red wire in picture) to the outside cage, to be used to (momentarily) ground the inside cage in the event it accidentally gets charged.
  • Set the electrometer to 30V full-range, and to measure 0V at half-range.
  • Remove the top hemisphere.
  • Connect the bottom hemisphere to the outside of the cage (blue wire in picture).
  • Connect the positive 1000V terminal of the power supply to the inner sphere (yellow wire in picture).
  • Do not connect the ground terminal of the power supply to anything. Power supply and electrometer share a ground through the power cord.
  • Raise the inner sphere to 1000V potential.
  • Ground the proof plane, the proof plane handle, and your fingers by touching each of them to the outside cage.

At this point the bottom hemisphere is grounded and the inner sphere is at 1000V. Make some measurements to gain confidence:

  • Touch the proof plane to the inner sphere
  • Bring the proof plane inside the inside cage. Electrometer needle should deflect positive by about 6V. This indicates there is positive charge on the inner sphere
  • Ground the proof plane on the outside cage. Touch the proof plane to the outside of the bottom hemisphere. Bring the proof plane inside the inside cage. Needle should not deflect. This indicates there is no charge on the outside of the bottom hemisphere.
  • Ground the proof plane on the outside cage. Touch the proof plane to the inside of the bottom hemisphere. Bring the proof plane inside the inside cage. Needle should deflect negative almost 6V. This indicates there is negative charge on the inside of the outer hemisphere.
  • Disconnect the yellow wire from the inner sphere connector. Turn off the power supply.
  • Ground the proof plane on the outside cage. Touch the proof plane to the inner sphere. Bring the proof plane inside the inside cage. Needle should deflect, almost as much as before.

Here is the actual Cavendish Experiment:

  • Replace the outer upper hemisphere.
  • Disconnect the grounding wire (ie the blue lead) from the outside cage, and touch it momentarily to the inner sphere connector. This brings the two spheres to the same potential.
  • Replace the grounding wire (the blue lead) on the outside cage. This grounds the outer sphere.
  • Remove the outer top hemisphere.
  • Ground the proof plane on the outside cage. Touch the proof plane to the inner sphere. Bring the proof plane inside the inside cage. The needle should not deflect!

Discussion: The fact that the needle does not deflect on the last measurement shows that the inner sphere has been left neutral, and was proposed by Cavendish as a consequence of the inverse square law of the electrostatic force. If the electrostatic force dropped off faster than inverse square, the inner and outer spheres would end up with charges of the same sign. If the electrostatic force dropped off slower than inverse square, the inner and outer spheres would end up with charges of opposite sign. Only if the electrostatic force drops off as inverse square can the inner sphere end up with no charge.

Cavendish's account of his experiment may be found in "The Electrical Researches of Henry Cavendish", edited by James Clerk Maxwell, paragraphs 217-234.

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