Faraday's ice pail experiment is a simple electrostatics experiment performed in 1843 by British scientist Michael Faraday that demonstrates the effect of electrostatic induction on a conducting container. For a container, Faraday used a metal pail that had originally been made to hold ice, which gave the experiment its name.John Ambrose Fleming, The experiment shows that an electric charge enclosed inside a conducting shell induces an equal charge on the shell, and that in an electrically conducting body, the charge resides entirely on the surface. It also demonstrates the principles behind electromagnetic shielding such as employed in the Faraday cage. The ice pail experiment was the first precise quantitative experiment on electrostatic charge. It is still used today in lecture demonstrations and physics laboratory courses to teach the principles of electrostatics.
Description of experiment 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:
"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 be positive, E will also diverge positively; if C be taken away, E will collapse perfectly... As C enters the vessel A the divergence of E will increase until C is ... below the edge of the vessel, and will remain quite steady and unchanged for any greater depression. This shows that at that distance the inductive action of C is entirely exerted upon the interior of 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 and C, upon being withdrawn, ...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: The experiment uses a conductive metal container A open at the top, insulated from the ground. Faraday employed a 7 in. diameter by 10.5 in. tall pewter pail on a wooden stool,(B) but modern demonstrations often use a hollow metal sphere with a hole in the top, or a cylinder of metal screen, mounted on an insulating stand. Its outside surface is connected by a wire to a sensitive electric charge detector. Faraday used a gold-leaf electroscope, but modern demonstrations often use a modern electrometer because it is far more sensitive than an electroscope, can distinguish between positive and negative charge, and gives a quantitative readout., p.4-5 The container is discharged by connecting it briefly to a large conducting object, called a ground (earth); this can be done by touching it with a finger, using the conductive human body as a ground. Any initial charge drains off into the ground. The charge detector reads zero, indicating that the container has no charge. A metal object C (Faraday used a brass ball suspended by a nonconductive silk thread, but modern experiments often use a small metal ball or disk mounted on an insulating handle) is charged with electricity using an electrostatic machine and lowered into the container A without touching it . As it is lowered the charge detector's reading increases, indicating that the outside of the container is becoming charged. Once the object is well inside the lip of the container the charge detector levels off and registers a constant charge, even if the object is lowered further. The charge on the outside of the container is the same polarity as that on the object. If the charge detector is touched to the inside surface of the container, it is found to be charged with opposite polarity. For example, if the object C has a positive charge, the outside of the container A will be found to have a positive charge, while the inside of the container has a negative charge. If the object C is moved about inside the container without touching the walls, the charge detector's reading will not change, indicating that the charge on the outside of the container is not affected by where the charged object is inside the container. If the charged object C is lifted out of the container again, the charge detector will decrease to zero again. This shows the charges on the container were induced by C, and the container has no net charge. Therefore, the opposite charges induced on the inside and the outside must be equal in size. The charged object C is touched to the inside of the container. The charge detector reading does not change. However, if the object is now withdrawn from the container, the reading stays the same, indicating that the container now has a net charge. If the object is then tested with the charge detector, it is found to be completely uncharged, and the inside of the container is also found to be uncharged. This indicates that all the charge on C has been transferred to the container, and has exactly neutralized the opposite charge on the inside surface of the container, leaving only the charge on the outside. So the charge on the inside of the container was exactly equal to the charge on C. Kits are available from educational supply firms containing all the apparatus needed for students to perform the experiment.
Preventing error due to stray charges Stray static electric charges on the experimenter's body, clothes, or nearby apparatus, as well as AC electric fields from mains-powered equipment, can induce additional charges on parts of the container or charged object C, giving a false reading. The success of the experiment often requires precautions to eliminate these extraneous charges: Any charges on the container and nearby conductive objects should be removed before the experiment by grounding (earthing); touching them briefly to some large conducting object called a ground. Any charge on the object will flow into the ground due to its mutual repulsion. This can be accomplished by touching them with a finger, using the conductive human body as a ground. However the experimenter's body itself should be grounded frequently by touching a good metal ground such as a metal workbench, or preferably a water pipe or the grounding wire of the building's mains power wiring. Ideally the experimenter's body should be grounded throughout the experiment. Some demonstration kits include conductive ground sheets which are laid on the workbench under the apparatus, and antistatic wrist straps the experimenter wears during the experiment, which are connected to a good ground. The electrometer measures charge with respect to ground, so it requires a connection to ground during use. It has a ground wire, usually colored black, ending in a clip which should be attached to a metal ground during use. The experimenter should avoid excessive movement during the experiment. Walking around or waving his/her arms can cause the buildup of static charges on clothing. The experimenter should hold the handle of the charged object C as far from the object and the container as possible when lowering the object into the container. In professional student laboratory kits, the container A is often in the form of two concentric cylinders of metal screen, open at the top. A screen acts the same as a solid metal sheet for electrostatic charge, as long as its holes are small. The inner cylinder is the Faraday pail container itself, separated from the outer cylinder with insulating supports. The outer cylindrical metal screen surrounds the inner, and acts as a ground to shield it from stray charges. This design largely eliminates the stray charge problem, as well as allowing the experimenter to see inside the container. The electrometer ground lead is clipped to the outer ground screen, and the experimenter touches this screen while performing any procedure. To ground the inner screen, the experimenter can bridge his finger between the inner and outer screens. When doing this it is important that he lift his finger first from the inner screen, not the outer, to avoid leaving charge on the inner screen. Charge can leak off the charged object C and the container along handles and supports due to surface layers of dirt and oil from fingerprints. If this is suspected the equipment should be washed with detergent to remove oils and dried. When measuring the charge on the inside or outside surface of the container, the charge detector should not be touched to the surface near the lip of the container. Extra charge concentrates near the edge of the opening due to the geometry of the metal.
Explanation
Conductive metal objects contain mobile electric charges (electrons) that can move about freely in the metal. In an uncharged state, each part of the metal contains equal quantities of positive and negative charges, intimately mixed, so no part of it has a net charge. If an external charged object is brought near a piece of metal, the force of the charge causes these internal charges to separate. The charges of opposite polarity to the external charge are attracted to it, and move to the surface of the object facing the charge. The charges of the same polarity are repelled and move to the surface of the metal away from the charge. This is called electrostatic induction. In Procedure 2 above, as the charge C is lowered into the container, the charges in the metal of the container separate. If C has a positive charge, the negative charges in the metal are attracted to it and move to the inner surface of the container, while the positive charges are repelled and move to the outside surface. If C has a negative charge, the charges have opposite polarity. Since the container was originally uncharged, the two regions have equal and opposite charges. The induction process is reversible: in Procedure 4, when C is removed, the attraction of the opposite charges cause them to intermingle again, and the charge on the surfaces reduces to zero.
It is the electrostatic field of the charged object C which causes the mobile charges to move. As the charges in the metal separate, the resulting regions of induced charge on the surfaces of the metal container create their own electrostatic field, which opposes the field from C. The field of the induced charges exactly cancels the field from C throughout the interior of the metal. The electrostatic field inside a piece of metal is always zero. If it was not, the force of the field would cause more motion of charges and more charge separation, until the electric field became zero. Once C is well inside the container, almost all of the electric field lines from C strike the container surface. The result (proved below) is that the total charge induced on the inside of the container is equal to the charge on C.
In Procedure 5, when C is touched to the container's inner wall, all the charge on C flows out and neutralizes the induced charge, leaving both the inner wall and C uncharged. The container is left with the charge on its outside. The net effect is that all the charge that was previously on C is now on the outside of the container.
An important conclusion that can be drawn from this is that the net charge inside a closed conducting container is always zero, even if a charged object is put in. If the charge inside can find a conducting path to the container wall, it will flow to the outside surface of the container due to its mutual repulsion. If it cannot, the interior charge will induce an equal and opposite charge on the inside surface, so the net charge inside is still zero. Any net charge on a conducting object is located on its surface.
Proof induced charge is equal to object's charge