Description: Portable electrometer.

Caption: A small lever balance or the "idiostatic balance" that produces the fine measurement is in the bottom of the leyden jar and is part of the attracted plate. This part of the instrument was very small and required a magnifying lens for the operator to read the scale. The ''idiostatic balance" has one end of the lever fixed and the other end free to move in a hole in the attracted plate.

Introduction: This instrument is a form of attracted disc electrometer. It has an attracted plate surrounded by a "guard ring" at the bottom of the glass leyden jar and a moveable attracting plate in the top of the jar. The position of the attracting plate is read from a vernier scale inside the leyden jar. Reducing the distance between the plates reduces the range of the instrument that was about 0.5 -5000v maximum.

Background: When obtaining a measurement the distance between the plates is directly proportional to the applied voltage so the gap is reduced until the idiostatic balance just reads zero. This type of instrument was used for atmospheric measurements during the 1900-1904 National Antarctic Expedition.

Reference: Green, George & Lloyd, John T.. "Kelvin's Instruments and the Kelvin Museum", 1970 p. 59

Reference: Holbrook, Mary. "Science Preserved A Directory of Scientific Instruments in the collections in the United Kingdom and Eire", 1992 p. 132

Historical Context: Design of the Coulomb balance - When: 1784 - Where: France - Who: Coulomb, Charles Augustin de (1736-1806) French physicist

Historical Context: Design of this form of the attracted disc electrometer - Who: Thomson, William, Professor (1824-1907) Sir William Thomson from 1866 and Ist Baron Kelvin of Largs from 1892 better known as Lord Kelvin

Historical Context: Design of this form of the attracted disc electrometer by Lord Kelvin - When: 1867 - Where: Glasgow, Scotland

Historical Context: Designed by William Thomson, Lord Kelvin, Professor of Natural Philosophy, University of Glasgow - Where: Glasgow, Scotland

Historical Context: Invention of the attracted disc electrometer - Who: Harris, William Snow, Sir (1791-1867) Physician, gentleman scientist and electrician

Historical Context: Invention of the attracted disc electrometer by Harris - When: 1834 - Where: England

Historical Context: Manufacture - When: circa 1870 - Where: Glasgow, Scotland - Who: James White & Company

Historical Context: Publication of the invention of Gilbert's "versorium" in "De Magnete" - When: 1600 - Where: London, England

Historical Context: Publication of the ivention of the electroscope Gilbert called a "versorium" in "De Magnete" - Who: William Gilbert (1540-1603) English physician

Historical Context: Used at the Gilbert Scott Buildings in lecture demonstrations in the Department of Natural Philosophy - Who: Thomson, William, Professor (1824-1907) Sir William Thomson from 1866 and Ist Baron Kelvin of Largs from 1892 better known as Lord Kelvin

Historical Context: Used in the Gilbert Scott B'lding by Prof. William Thomson in the Department of Natural Philosophy now called the Department of Physics and Astronomy - When: circa 1870-1899

Historical Context: Event - "Lord Kelvin: A life in science" exhibition, Hunterian Museum, University of Glasgow. Object on display in this exhibition.