Faraday's bag is used to show that electric charge resides on the surface of a conductor. The example from Colby College at the right shows the original form: A muslin bag is attached to a metal hoop supported by an insulating glass rod. An electric charge is placed on the interior of the bag, and reappears on the outer surface of the bag. Its presence there can be demonstrated with a proof plane and electroscope. If the bag is then turned inside out by pulling on the strings, the charge appears on the new outside of the bag. This is clearly a version of the Faraday Ice Pail experiment.
Another version is the apparatus at the left above, from the Harvard University Collection of Historical Instruments. A metallic ribbon is wound around the drum, and, along with the drum and all of the other metallic parts, forms a single-conductor capacitor. A charge is then placed on the system, and an attached electrometer diverges. Now the (missing) crank is used to unwind the ribbon, increasing the surface area of the system. Since capacitance is proportional to surface area, the capacitance increases. The total charge on the system remains the same, and since V = Q/C. the potential of the system decreases, causing the electroscope leaves to come closer together.
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