Quadrant Electrometer

   The first indicators of electric potential were Electroscopes, which gave only rough and non-linear indications of the potential. William Thomson (1824-1907; Lord Kelvin) developed the basic Quadrant Electrometer in 1867. This allowed absolute measurements to be made of electrostatic potentials.

   Inside the four-part brass box a light aluminum paddle hangs from a torsion fiber. This paddle has been charged by the Leiden jar in the bottom of the apparatus. This jar is filled with sulphuric acid. Each brass quadrant in the box is electrically connected to the one opposite it. The wires from the potential difference to be measured are connected to the pairs of quadrants, and the angle through which the paddle swings is proportional to the potential difference. The Faraday cages over the mechanism gives the apparatus its name of bird-cage electrometer.

   The two electrometers were made by Griffin of London (on the left) and Elliott of London (on the right).
                  St. Patrick's College, Maynooth, Ireland                                                 Kenyon College
    In the 1880's Lord Kelvin designed quadrant electrometers for use in the electrical power industry. These were made by White of Glasgow and are shown below. In the United States, these were imported by James W. Queen of Philadelphia, and sold for $100.80 in the 1887 Catalogue and Price-List of Electrical Testing Apparatus. Queen referred to them as electrostatic voltmeters. 

   The apparatus at the left is at Yale and that at the right is at Glasgow.


 The quadrant electrometer at the left was purchased by the Vanderbilt University during the academic year 1890-90. Although there are no maker's mark on it, the minutes of the Vanderbilt Board of Trustees note that it was made by Edelmann of Stuttgart. 

   The apparatus is currently (2001) on display in the Garland Collection of Classical Physics Apparatus at Vanderbilt.

   At the right is the same piece of apparatus, this time in the collection at Dartmouth College. In both cases the suspension wire is broken, a standard feature of surviving quadrant galvanometers

   And then there is the humble little quadrant electrometer at Kenyon, with no maker's name. The electrostatic shielding is provided by the tinfoil lining the wooden case. The punched metal front at the left has a hole in it to let light reflect from the rotating mirror attached to the suspension. 

   Lord Kelvin's Multicellular Voltmeter (below) is a rather unusual research instrument in the collection of the University of Vermont. In effect, it is a series of quadrant electrometers connected in series and hung, one below the other, from the suspension fiber. The digram below at the right (from R. Mullineux Walmsley, Electricity in the Service of Man (Cassell, London, 1904), pg 622) shows the cellular construction. This apparatus is also discussed in the website of the University of Queensland, http://www.physics.uq.edu.au/physics_museum/electricity.html#cat037

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