A patent electrical balance J. White Glasgow, late 19th century With two pairs of substantial coils flanking copper and brass assembly set behind a scale carrying a slider and calibrated in single units labelled in two’s from 4 to 50 over second fine scale also calibrated in single units labelled every ten divisions from 0 to 67, the whole mounted on heavy ebonite base applied with silvered oval trade plate engraved Sir W.m Thomsons, Patent, ELECTRICAL BALANCE, 73, J. WHITE GLASGOW to rear edge, with brass framed glazed cover inscribed No. 73 to front and raised on three fine screw adjustable brass feet with substantial laminated copper connection terminal to rear, 53.5cm (21ins) wide. James White is recorded in Gloria, Clifton Diorectory of British Scientific Instrument Makers 1550-1851 as worrking from numerous addresses in Glasgow 1850 to beyond 1900, he was appointed instrument-maker to the University of Glasgow. This form of current balance was used as a secondary standard of electric current. Thomson’s earliest form dates from 1882 and a more accurate one from 1887. Parallel electric currents attract or repel each other, depending on whether the currents are in the same or opposite directions. In this instrument there are two pairs of fixed coils and between each of the pairs is a moveable coil. The fixed coils are connected in opposition so that one attracts and the other repels the moving coil to increase the sensitivity. Current sent through the coils causes the balance beam to tilt. The balance is restored by placing one of the calibrated weights in the V-shaped trough on the right and by moving the slider along the graduated scale. Strings are provided to move the slider even when the protective glass cover is in place. Since the position of the slider is proportional to the square of the current, the scale is marked off quadratically. Different current ranges are available by using different weights. Five models were available which covered the range from 0.01 to 2500 amperes. Alternating currents can be read as well as direct currents. Thomson current balances were widely used for calibrating other instruments but by World War I they were superseded by more convenient systems involving springs rather than by utilizing gravity.
A patent electrical balance J. White Glasgow, late 19th century With two pairs of substantial coils flanking copper and brass assembly set behind a scale carrying a slider and calibrated in single units labelled in two’s from 4 to 50 over second fine scale also calibrated in single units labelled every ten divisions from 0 to 67, the whole mounted on heavy ebonite base applied with silvered oval trade plate engraved Sir W.m Thomsons, Patent, ELECTRICAL BALANCE, 73, J. WHITE GLASGOW to rear edge, with brass framed glazed cover inscribed No. 73 to front and raised on three fine screw adjustable brass feet with substantial laminated copper connection terminal to rear, 53.5cm (21ins) wide. James White is recorded in Gloria, Clifton Diorectory of British Scientific Instrument Makers 1550-1851 as worrking from numerous addresses in Glasgow 1850 to beyond 1900, he was appointed instrument-maker to the University of Glasgow. This form of current balance was used as a secondary standard of electric current. Thomson’s earliest form dates from 1882 and a more accurate one from 1887. Parallel electric currents attract or repel each other, depending on whether the currents are in the same or opposite directions. In this instrument there are two pairs of fixed coils and between each of the pairs is a moveable coil. The fixed coils are connected in opposition so that one attracts and the other repels the moving coil to increase the sensitivity. Current sent through the coils causes the balance beam to tilt. The balance is restored by placing one of the calibrated weights in the V-shaped trough on the right and by moving the slider along the graduated scale. Strings are provided to move the slider even when the protective glass cover is in place. Since the position of the slider is proportional to the square of the current, the scale is marked off quadratically. Different current ranges are available by using different weights. Five models were available which covered the range from 0.01 to 2500 amperes. Alternating currents can be read as well as direct currents. Thomson current balances were widely used for calibrating other instruments but by World War I they were superseded by more convenient systems involving springs rather than by utilizing gravity.
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.jpg "Source: Christie's")
