Differential Galvanometer
   The basic two-coil differential galvanometer is illustrated by the four pieces below. The construction can be best seen in the galvanometer at the right below, built by Prof. Joseph Naylor of DePauw University in Greencastle, Indiana. The small compass needle is suspended from a fiber attached to the tube at the top. The tube is missing from the example from Hampden-Sydney, although the stub of the suspension wire is visible. Identical coils are placed on either side of the needle. When these are connected to different circuits, the needle responds to the difference in the two currents. Antoine Cesar Becquerel (grandfather of the discoverer of radioactivity) devised the differential galvanometer in 1825. Instead of a single needle, a series of parallel needles, shaped to fit the space between the two coils, was often used for increased sensitivity.

   The instrument at the left below is marked Queen of Philadelphia. The Hampden-Sydney instrument in the second row is by A.P. Gage, while the one from Denison is by E.S. Ritchie, both of Boston.
                                   DePauw University                                                             DePauw University
 
                             Hampden-Sydney College                                                             Denison University

      The three instruments below use astatic needles to cancel the effects of the earth's magnetic field. The picture in the middle shows a closeup of the interior of an instrument which is just like the one at the left. Two parallel groups of magnetic needles are attached to the torsion fiber, with the upper set pointing in the opposite magnetic direction to the lower set. The net magnetic moment of the needles is thus zero, and they do not respond to the earth's magnetic field. Unlike the simple form of astatic galvanometer, both sets of needles are in the middle of a pair of coils. On one side, the upper and lower coils are connected in series, with the current passing through them in opposite directions. Thus, there is twice the torque on the needles. The coils on the other side can be connected to a separate circuit, thus making the instrument a differential galvanometer. Or, they can be connected in series with the other coils, thus doubling the sensitivity of the instrument once more.
 
            Duke University                                             Denison University                                      Denison University

   The Duke instrument is marked "Willyoung", but is clearly the same as the Leeds and Northrup instrument in the center picture. Elmer Willyoung sold his business to Morris E. Leeds in 1893, and in 1903 Leeds joined forces with Edwin Northup to form Leeds & Northrup.The cost in the 1903 L & N catalogue was $65. The Denison instrument at the right is by Ritchie. All three instruments have a magnetic compensating bar (below the base in the Leeds and Northup galvanometers and on top in the Ritchie galvanometer), allowing any residual effects of the earth's magnetic field to be offset.

   The small motions of the needles are amplified by means of a beam of light reflecting from the small mirror between the two sets of needles.
 
   At the left is a beautiful differential galvanometer from the University of Vermont in Burlington. 

   It was made by Latimer, Clark and Muirland & Co, Ltd of Westminster [London], and is the only apparatus by this company I have ever seen in the United States. 
   When I spent a day in March 2001 photographing portions of the Garland Collection of Classical Physics Apparatus at Vanderbilt University, I photographed quite a number of galvanometers. I think that this differential galvanometer is the one described in the 1983 catalogue to the collection, written by Prof. Robert T. Lagemann as: 

   "Galvanometer: Standing 21 cm high and supported on three adjustable brass legs, this galvanometer is marked "Th[?] A. DUBOSCQ / A PARIS / QUEEN & CO. SOLE AGENTS / in PHILADELPHIA". A Central column supports the end of the thread or wire suspending the needle."
   This differential galvanometer by Morris E. Leeds of Philadelphia I photographed at the Unversity of Mississippi in November 2002. It is part of the Millington/Baranrd Collection in the University Museum.

   The apparatus was made between 1898 and 1903. 








   The unmarked differential galvanometer at the right is in the apparatus collection at Case Western Reserve University in Cleveland, Ohio.

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