Faraday's ice pail experiment

Faraday's ice pail experiment is a simple electrostatics experiment performed in 1843 by British scientist Michael Faraday[1][2] that demonstrates the effect of electrostatic induction on a conducting container.

For a container, Faraday used a metal pail made to hold ice, which gave the experiment its name.

[4][5] It also demonstrates the principles behind electromagnetic shielding such as employed in the Faraday cage.

[8] It is still used today in lecture demonstrations and physics laboratory courses to teach the principles of electrostatics.

[9] Faraday's description of the experiment, from a letter he wrote on February 4, 1843 to Richard Phillips, the editor of Philosophical Journal, and published in the March 1844 issue:[1][10] "Let A in the diagram represent an insulated pewter ice-pail...connected by a wire to a delicate gold-leaf electrometer E, and let C be a round brass ball insulated by a dry thread of white silk, three or four feet in length, so as to remove the influence of the hand holding it from the ice-pail below.

Let A be perfectly discharged, and then let C be charged at a distance by a [electrostatic] machine or Leyden jar, and introduced into A..

If C is made to touch the bottom of A, all of its charge is communicated to A, ... and C, upon being withdrawn, ... is found to be perfectly discharged."

Below is a detailed modern description of the experimental procedure:[3][4][6][9][11] Kits are available from educational supply firms[13] containing all the apparatus needed for students to perform the experiment.

The success of the experiment often requires precautions to eliminate these extraneous charges: Conductive metal objects contain mobile electric charges (electrons) that can move about freely in the metal.

[7][9][19] Assume the container A completely encloses the object C, without an opening (this assumption is explained below), and that C has a charge of Q coulombs.

[12][21] The field lines extending from it end on charges induced in the walls or other objects in the room.

In his experiment, Faraday closed the opening by attaching the metal lid of the pail to the thread suspending the ball, so when the ball was lowered to the center of the container the lid covered the opening.

The experiment works very well even for containers with large uncovered openings, like Faraday's pail.

John Ambrose Fleming, a prominent early electrical researcher, wrote in 1911:[3] .

it is curious to note how large an opening can be made in a vessel which yet remains for all electrical purposes a 'closed conductor'.But the experiment is often explained, as in the above sections, by assuming the container has no hole.

Since there is no electric field in the intervening volume of the metal, the charge distribution on the outside surface of the container and its electric field is completely unaffected by the charges inside the container.

[9][11] If the charged object inside the container is moved about as in Procedure 3, the induced charge distribution on the inside surface will redistribute itself, maintaining the cancellation of the electric fields outside the inner surface.

[4] This is how charge is transferred to the top terminal of a Van de Graaff generator.

[4][7] The terminal is a hollow metal shell and functions as a Faraday pail.

In his original 1844 paper, Faraday also investigated the effect of using several conducting containers one inside the other.