The Balance in Measurement.
Our grandfathers supposed that trade began in barter; we have been able to go one step further back in history to find that barter followed upon the custom of exchanging presents. This custom, among shrewd and self-respecting people, came at length to a degree of fairness, and led to rough and ready modes of weighing, gradually improved. In the British Museum, in a papyrus of Hunnafer, who lived in Egypt thirty-three centuries ago, we have pictured a well-constructed balance of equal arms, in which a feather is outweighing a human soul. In its successive improvements the balance registers the progress of many arts and sciences, and in its turn has promoted them all. It must be built of a metal, or an alloy, hard, durable, and not easily corroded. Its centre of motion should be a little above its centre of gravity; its knife edge should have an angle of about 60 degrees. Appliances must render it easy to lift the weighing apparatus when out of use, so that unnecessary wear of the knife edge may be avoided, as well as needless strain throughout the structure. Air currents should be kept off by a suitable case, or, better still, the instrument should be enclosed in a receiver exhausted of air altogether. The weights, made with scrupulous care of standard metal or alloy, should be guarded from tampering, abrasion, and corrosion, from dirt or other accretions. A weighing should be slowly performed, the weights placed in the center of one pan, the object weighed in the center of the other pan; to eliminate errors due to inequality in the length of arms, the article weighed and the weights are then made to exchange places. The platform should be of the utmost strength and rigidity, so as precisely to maintain its level at all times.
A Rueprecht balance.
As long ago as 1798 a balance was erected having an accuracy of one part in 1,600,000; fifty years later ten-fold greater accuracy had been attained; to-day results much more astonishing are achieved. A [precision balance] manufactured by Messrs. Albert Rueprecht & Son, Vienna, is shown on page 220, as furnished in 1902 to the International Bureau of Weights and Measures at Sevres, France. It is provided with means for applying the smallest weights of platinum from a distance of three to four metres, so as to guard against perturbations due to the warmth of an operator’s body. The weights may be shifted from one pan to the other, and the oscillations observed through a telescope, at a distance of four metres. This balance will detect the 1⁄500 of a milligram when weighing a mass of 500 grams, or one part in 250,000,000. Such balances, and those of Paul Bunge, of Hamburg, require ten to twenty months of skilled labor for their completion. The International Bureau of Weights and Measures has a balance of extraordinary sensitiveness at the Pavillon de Breteuil, Sevres, where the work of the Bureau goes forward. This instrument measures the difference in the attraction of the earth for a mass of one kilogram when that weight is moved nearer to or farther from the centre of the earth by as little as one centimetre. Thus placing two weights, of common shape, each a kilogram, one on top of the other, and two other weights in the other pan beside one another, would introduce a noteworthy difference in a comparison.