Description: Low resistance bridge.

Caption: This instrument uses a standard resistance coil as a reference, the net is then made up of the open spring coils that can be varied in length without changing their resistance, a length of copper conductor of known conductivity, and the unknown sample. The copper conductors are clamped into the apparatus against a scale measured in centimeters so that the length of conductor in the circuit can be measured.

Introduction: This instrument, made in about 1860, is a modified form of the Wheatstone Bridge. It was designed by Lord Kelvin in about 1855. It was used to calculate the conductivity of samples of copper intended for use in the transatlantic cable by the Wheatstone net method.

Background: Whilst working in the 1850's with his friend professor Peter Guthrie Tait (1831-1901), Kelvin found a large variation in the quality of the material used for making telegraph wires. Kelvin used conductivity, the inverse of resistance, to investigate the material and called his new unit the 'mho' which is still in use today. they discovered that some of the copper used was only as conductive as iron!.

Reference: Green, George & Lloyd, John T.. "Kelvin's Instruments and the Kelvin Museum", 1970 pp. 42,43 & 61

Reference: Holbrook, Mary. "Science Preserved A directory of scientific instruments in collections in the United Kingdom and Eire", 1992 p. 132

Historical Context: Design of the double bridge by Kelvin - When: 1860 - Where: Glasgow, Scotland - 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 the single restoration bridge - Who: Christie, Samuel Hunter (1784-1864)

Historical Context: Design of the single restoration bridge by Hunter - When: 1833

Historical Context: Design of the single restoration bridge by Hunter, Woolwhich Military Academy - Where: London, England

Historical Context: Manufacture - When: circa 1860 - Where: Unknown, the instrument is unsigned

Historical Context: Publication of Hunter's design of restoration bridge - Where: London - Who: Sir Charles Wheatstone (1802-1875) English physicist

Historical Context: Publication of Hunter's design of restoration bridge by Wheatstone - When: 1843

Historical Context: Used by Lord Kelvin to determine its resistance of copper conductors - When: circa 1860 - Where: University Avenue, Glasgow, Scotland - 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: Event - "Lord Kelvin: A life in science" exhibition, Hunterian Museum, University of Glasgow. Object on display in this exhibition.