mass The traditional and contemporary scheme for weighing gold and silver, in recent times extended to platinum and other precious metals. The widely familiar ounce used for reporting the price of gold, rarely qualified, is the ounce of the troy scale.
Unlike the more usual avoirdupois scale with its pound of 16 ounces and 7 000 grains (making its ounce of 437.5 gr), the troy scale, like the apothecaries' scales, has a pound of only 12 ounces each of 480 grains, giving a total of 5 760 grains, the grain being the one unit common to all three scales. The distinctive scale for troy units is shown in Table 56.
| BI-troy, US-C-troy | Internat values: | SI | US-C-av | |||
|---|---|---|---|---|---|---|
| grain | … | … | … | 64.8~ μg | 1 gr | |
| 24 | pennyweight | … | … | 1.56~ g | 24 gr | |
| 480 | 24 | ounce | 31.1~ g | 1.10~ oz | ||
| 5 760 | 288 | 12 pound | 373.~ g | 13.2~ oz |
As with the avoirdupois units, the troy units have for centuries been very close to their current international value, probably the same to at least six significant figures. Although for many centuries the scale of goldsmiths, etc., the troy scale became officially legal in the UK only with BI in 1825. A prototype troy pound actually provided the standard for all ‘weights’ in BI until 1855 and in the USA until the Mendenhall Order of 1893. Current values are based on the international grain, adopted in 1959, of 64.798 91 mg.
The troy pound was removed from official UK measures in 1878. The pennyweight was removed in 1970, and the grain in 1985.The UK Weights and Measures Act 1985 explicitly excluded from use for trade the bushell, cental, chain, drachm, dram, fluid drachm, furlong, grain, hundredweight, ounce apoth., peck, pennyweight, quarter, quintal, rood, scruple, stone, ton, the square mile, cubic inch, cubic foot, cubic yard, and the term ‘metric ton’. However, the legal status of the bushell, fluid drachm, and peck had been repealed, along with all apothecaries' units and troy units other than ounce, by Order in 1970. Besides the remaining BI units and the simple SI units, the Act included the kilometre, decimetre, centimetre and millimetre, the square metre, square decimetre, square centimetre and square millimetre, the hectare and decare along with the are, the cubic metre, cubic decimetre and cubic centimetre, the hectolitre decilitre, centilitre and millilitre, the tonne (or ‘metric tonne’), kilogram, hectogram, milligram and carat (metric). All had been included in the similar Act of 1963, but with some variation of name: -gram was -gramme, decare was dekare, the tonne appeared only as metric ton.
