Thomas Parker: Inch Weights

By Norman Biggs

A box of British Grain Weights.

The box of weights shown above is similar to two boxes illustrated in Equilibrium1 some years ago. The one shown here has the following inscription stamped underneath.

British Grain Weights decimalized and correlated on the inch length.

Unit = 250 grains = 1 cubic inch of water at 122º FAH.

1/1000 unit = ¼ grain.

On one of the EQM boxes there is a similar inscription engraved on a brass plate, together with the name Thomas Parker

Thomas Parker2

Thomas Parker was the epitome of Victorian self-help. Born into a poor family on 22 December 1843, by the age of 10 he was working in the famous ironworks at Coalbrookdale. His ambition led him to take evening classes, and to move around gaining experience of the industrial world. He was in Birmingham in 1863, worked in the Potteries and Manchester, and by 1868 he was back in Coalbrookdale as foreman of the works where he had worked as a child.

But that was only the beginning. He became an electrical engineer, designing one of the first battery accumulators, and then taking a major role in several of the earliest electric tramway and railway systems. For this work he was awarded the Stephenson Gold Medal of the Institution of Civil Engineers. In 1882 he founded an electrical engineering company in Wolverhampton, which produced accumulators and dynamos. Parker also invented Coalite. In 1908 he retired to Coalbrookdale, where he died in 1915. There is a memorial to him in Holy Trinity Church, Coalbrookdale.


The weights are intended to show how, in the imperial system, the unit of mass can be related to the unit of length. If the inch is a known quantity, then it is possible to define the grain so that 250 grains is the mass of a cubic inch of water at 122ºF. Thomas Parker explained this idea in a letter to The Times, 6 May 1908. His letter was also printed in the Journal of the British Weights and Measures Association3, for whom he wrote a pamphlet entitled The Inch, the Metre, and the Metric System. The boxes of British Grain Weights can therefore be confidently ascribed to him, and dated to around 1908.

Metrological background

The Weights and Measures Act of 1824 defined the inch and the grain in terms of actual objects, the standard yard and the standard troy pound. The Act also provided (Section V) that if the pound were lost, it should be reconstructed by using the rule that one cubic inch of water weighs 252.458 grains, if weighed in air, with brass weights, at 62º F, the barometer being at thirty inches.

The standard pound was in fact destroyed in the fire at the Houses of Parliament in 1834, but by then serious doubt had been cast4 on the accuracy of the figure of 252.458. Consequently the new standard pound was constructed, not by the rule given in the Act of 1824, but by comparison with surviving copies of the original. In 1889 it was thought desirable to give a new official value for mass of a cubic inch of water at 62º F, the value chosen then being 252.286 grains5. Curiously, in his letter to The Times in 1908, Parker gives a figure of 252.463 grains.

Whatever the figure should be, the principle is clear. Since water expands on heating, the mass of one cubic inch at higher temperatures is less than its value at 62º F. Parker claimed that his own experiments showed that at 122º F it is exactly 250 grains, and he pointed out that 122º F (=50º C) is exactly midway between the melting point of ice and the boiling point of water. That is the basis for the inch weights: elegant, but not entirely practical, since making measurements at 122º F is not easy.

Decimalisation and metrication

Parker was interested in coinage, as well as weights and measures. In 1901 he distributed aluminium patterns for a decimal coinage, based on dividing the pound into one thousand parts (approximately equal to a farthing) and declaring five of these parts to be a penny6. This particular system (often known as the mil system) had been the subject of much discussion in the nineteenth century7.

The decimal coins tell us that Parker was not a dyed-in-the-wool supporter of all things imperial. His inch weights were certainly approved by the British Weights and Measures Association, but his decimal coinage would have been more to the liking of their arch-enemies, the Decimal Association.


1. Equilibrium 1986/7, p.956 and p.1017.
3. Journal of the British Weights and Measures Association, July 1908, pp.14-15.
4. Airy, G.B. Extracts of Papers [relating to] Restoration of the Standards, 1841, p.6
5. Connor, R.D. The Weights and Measures of England, 1987, p.271.
6. Hill, C.W. Tom Parker's Pennies. Coin Monthly, January 1979.
7. Connor, ibid. pp.280-1.

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