Biot's Hemispheres
   "To demonstrate practically the distribution of electricity on the surface of a conductor, the following apparatus was contrived by Biot. A sphere of conductucting matter is [placed on an insulating rod], and two thin hollow covers, of brass or copper, are provided with glass handles, and correspond to the shape and magnitude of the conductor. The sphere is electrified, and the covers are then applied, being held by the glass handles. After withdrawing them from [the sphere], they are found to be charged with the same kind of electricity as was communicated to [the sphere], which will be found to have lost the whole of its charge, proving that it resided on the surface only." From Vol. I of Pike's Illustrated Descriptive Catalogue of Optical, Mathematical and Philosophical Instruments, published by Benjamin Pike, Jr. in New York in 1856.
   Jean-Baptiste Biot (1774-1862) is best known to physicists for his work on the Biot-Savart law (1820), the first description of the magnetic force exerted by a wire on a small permanent magnet. 

   The apparatus below is in the collection of Allegheny College. At the right is an unusual form of Biot's hemispheres at Vassar College, dating from about 1900. .

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