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Description: | The current balance, or ampere balance, was invented by Lord Kelvin. The function of this apparatus is to measure the existence, and thus strength, of an electrical current running through a circuit.
The basic construction of this machine comprises of two large coils of strongly conductive wire, two thin metal arms', and a reading scale. The current balance functions by passing the subject current through the two coils of wire, one of which is attached to the first of the two arms of the balance. The resultant magnetic force between these two copper coils is measured by the amount of weight needed on the second arm of the balance to keep it in equilibrium. The measurement of this force is used to calculate the magnitude of the current, which can be read from the accompanying scale. If a high level of accuracy is required, the exact current can be calculated using the square root table and ancillary weights supplied with each instrument.
Each individual current balance reads only a certain current range, as its measurements are dictated by the size and accuracy of the coils. Therefore there are early current balances that read on the scale of centi-amperes (100ths of an amp), deci-amperes (10ths of an amp) and whole amperes. The wattmeter, or electrodynamometer, can be described as a descendant of the current balance.
An ammeter measures the electric current at any given point flowing along a circuit (in amperes); while the voltmeter is designed to measure the potential difference between two points in a circuit. A galvanometer is a sensitive ammeter; it is also an instrument for detecting and measuring the existence or strength of electric currents, but of very low levels.
The Hunterian scientific instrument collection includes a considerable collection of current balances, electrodynamometers, voltmeters, ammeters, and galvanometers of various types and ages, the earliest examples being those patented by William Thomson (Lord Kelvin) and James White in Glasgow, from the late 19th century. Although 19th and early 20th century machines comprise the bulk of the catalogued material, there is also a sizeable collection of mid-20th century examples. | Subjects: | scientific instruments | Source: | Hunterian Museum | Address: | University of Glasgow,
University Avenue,
G12 8QQ | Creator: | Shan Macdonald | Contributor: | Glasgow University: Dept. of Natural Philosophy, and various donors | Identifier: | C-0092 |
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