Faraday's Bag

   Electric charge placed on a conducting body accumulates on the outside of the body. This is normally demonstrated with the Faraday Ice Pail experiment in which a hollow, metallic body (usually spherical) has a smallish aperture in the top through which charge can be placed on the inner surface. The charge immediately travels to the outer surface, leaving the inside of the sphere neutral. 

   This demonstration can also be done with a cloth bag mounted on an insulating stand. The cloth conducts well enough to allow the charge to travel from one surface of the bag to the other as it is turned inside out using the strings attached to both sides of the bag's vertex. The apparatus at the right is at the apparatus collection of Colby College in Waterville, Maine. It is unmarked, but looks very much like an illustration in the 1860 catalogue of Edward S. Ritchie of Boston. The catalogue description is "Faraday's Muslin Bag; sustained upon an insulated ring of wire, with silk strings,....$4.00."


   A new muslin bag, and this example of Faraday's bag in the apparatus collection of Case Western Reserve University in Cleveland, Ohio would be as good as new.

   It was made by Queen of Philadelphia, and is listed in the 1888 catalogue at $4.00. The ring is six inches in diameter. 

   The apparatus at the left is from Miami University in Oxford, Ohio, and also serves to show that the charge on a metallic conductor remains on the outer surface. The two suspended spheres remain in contact as charge is added to either the inside or the outside of the open-ended cylinder, showing that there is no charge inside. The apparatus originally had two other spheres hanging from the post at the top. These touched the outside of the cylinder when it was uncharged, but flew out as soon as charge as added to the system. 

   At the right is a similar piece of apparatus from the demonstration room collection of Kenyon College. 

  The unmarked apparatus at the right I found sitting on a shelf in the demonstration room at the physics department at the University of Cincinnati. Flexible wire mesh is supported by three insulating rods held in rather heavy iron bases. Ribbons on both sides act as electrometers to indicate the presence of charge on the part of the mesh that is on the outside. 

   A picture of a similar device in the 1900 Max Kohl catalogue says that it was invented by Rosenberg and Kolbe. Otherwise, I have never seen it before. It ought to be revived for use in the lecture hall. 

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