Faraday Ice Pail 5B20.10

This demonstration repeats Michael Faraday's classic experiment of 1843, demonstrating electrostatic induction (first part) and the fact that charge resides on the outside surface of a hollow conductor (second part).

Equipment:

• The Faraday Ice Pail
• Brass ball and string

• 1000V DC power supply are in the Electrostatics Cabinet, B2.

• Electrometer is in Electrostatics Cabinet, B1.

Set-up:

• Connect the electrometer to the Ice Pail, grounding the base, as shown.
• Set the electrometer to center zero, and 10V.
• Set the 1000V power supply to 600V, and install an alligator clip on the "+" terminal.
• Discharge the Ice Pail by laying a finger on both terminals of the banana plug at the end of the coaxial cable.
• Remove your hand from the ground and bring it near the Ice Pail. If the electrometer needle deflects, grasp the Ice Pail, then release it and lay a finger (of the same hand) on the banana plug again.
• Repeat this process until the presence of your hand near the electrometer does not disturb the electrometer.

Experiment (first part):

• Take up the ball so that it is suspended from your hand by about 20 cm of string.
• Touch the ball to the end of the coax cable that is connected to the electrometer, thus grounding it.
• Carefully lower the ball into the Ice Pail, without touching the sides. The needle should not deflect.
• Carefully raise the ball out of the Ice Pail, without touching the sides.
• Touch the ball to the alligator clip, thus raising it to 600V potential.
• Carefully lower the ball into the Ice Pail. The needle should deflect to about 9.5 V as the ball is entering the Ice Pail, then remain at constant deflection while the ball is inside.
• Carefully raise the ball out of the Ice Pail. The needle should return to zero.

Experiment (second part):

• Touch the ball to the alligator clip, thus raising it to 600V potential.
• Carefully lower the ball into the Ice Pail, and allow it to rest on the bottom. The electrometer needle should deflect as before, and remain steady as the ball touches the Ice Pail.
• Carefully raise the ball out of the Ice Pail.
• While holding the ball suspended in midair, ground the Ice Pail by touching both terminals of the banana plug with your finger, as before.
• Again lower the ball into the Ice Pail. The needle should deflect very little, if at all.

Troubleshooting:

• If the electrometer deflects as the voltage on the power supply is varied, there is some leakage in the system which will compromise the experiment. The experimenter should not move his or her feet while doing this experiment.

Reference:

http://physics.kenyon.edu/EarlyApparatus/Static_Electricity/Faradays_Ice_Pail/Faraday_Ice_Pail.html

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