Description: Moving iron current balance.

Caption: The instrument is based on Faraday's observation that a mass of iron in a non-uniform magnetic field tends to move from areas of relatively weaker magnetic attraction to areas of stronger magnetic attraction. For this reason the coil is conical since the strength of the electromagnetic field of the coil is proportional to the number of turns of the coducting wire.

Introduction: This is prototype moving iron current balance was designed by Kelvin for switchboard use in electrical lighting and power systems. It "weighed" the force of electromagnetic attraction that drew an iron rod into a solenoid (a coil of conducting wire), when the current was on.

Background: In 1880 the inventor of the incandescent lamp Joseph Swan asked Kelvin to become a consultant to his new company. He refused the offer but was so impressed by the lamps that he immediately laid plans for installing a lighting system of his own. He then developed a range of safety features and measuring instruments, including the electric fuse, for the newly emerging electrical industry.

Reference: Green, George & Lloyd, John T.. "Kelvin's Instruments and the Kelvin Museum", 1970 pp. 36 & 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 - 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 by Lord kelvin - When: circa 1884

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

Historical Context: Manufacture - When: circa 1884 - Where: 16, 18, and 22 Cambridge Street, Glasgow, Scotland - Who: James White & Company

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 Lecture demonstrations in the Department of Natural Philosophy now called the Department of Physics and Astronomy - Where: Natural Philosophy Lecture Theatre, Gilbert Scott Building, University of Glasgow, University Avenue, Glasgow, Scotland

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 1895-1899

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