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.
Measurement of Time.
At the very dawn of civilization, the day, however crudely, was divided into parts. These parts, long afterward, probably in Babylonia, became the twenty-four hours which have descended to us. The means of time-keeping came first, in all likelihood, from measuring the simple shadow of a stick, the gnomon, still set up as a sun-dial in our gardens. Next came an hour-glass with its falling sand; the clepsydra, with its water dropping from a jar; the burning of candles definite in length. At last came the supreme discovery that a pendulum, of given length, if kept in one place oscillates in an unvarying period, be its arc of motion long or short. Tradition has it that in Arabia, about the year 1000 A. D., the pendulum was used in time-keeping. Granting this to be true, we must nevertheless give Galileo credit for his independent discovery as he observed the swaying lamp of the cathedral at Pisa, early in the seventeenth century. In 1657 Huygens employed a pendulum in the construction of a clock which, of course, displayed a new approach to accuracy. In 1792 Borda and Cassini had improved their time-pieces so as to be correct within one part in 375,000, that is to one second in 104 hours. For the sake of portability, clocks were gradually reduced in size until they became watches. Instead of a pendulum they were furnished with its equivalent, a balance wheel, Pierre Le Roy having discovered that there is in every spring a certain length where all the vibrations, great or small, are performed in approximately the same period. For actuation, watches were provided with mainsprings which have steadily undergone improvement in quality and in placing.
Time-Pieces Improved.
Many refinements have brought the time-keeper for the ship, the observatory, the railroad, to virtual perfection. Its wheels, pinions, balance-staffs are manufactured automatically, as at Waltham, Massachusetts, to an accuracy of 1⁄5000 inch or even less, thanks to that great inventor, Mr. Duane H. Church. In modern watch-making the most durable materials are used, magnetic perturbations are avoided by employing alloys insensitive to magnetism, and the effects of fluctuating temperatures are withstood by Earnshaw’s compensated balance wheel. This wheel is in halves, each nearly semicircular and attached at one end to a stout diameter. Its outer rim, being made of brass, when warmed expands more than its inner rim of steel. Thus, in a rising temperature the wheel curves inward with its duly placed weights, so that the reduction in elasticity of the hair-spring caused by heat is compensated. Experiments are afoot which look toward a marked improvement in the making of time-pieces, by using invar, a nickel-steel with practically no expansibility by heat. This alloy is already employed for pendulums with satisfactory results, both at the Naval Observatory and at the Bureau of Standards, in Washington. It has been described on [page 169].
Earnshaw compensated balance wheel for watches.
The Best Clocks in the World.
At the Paris Observatory the standard clock, by Winnerl, is in a vault twenty-seven metres underground. At that depth the temperature changes are less than one fifth of a degree during the year, yet the effect of barometric changes on the rate of the clock have proved to be serious. This difficulty is avoided in the Naval Observatory at Washington, by enclosing the standard clock in an air-tight case within which the air is reduced to a pressure lower than that ever shown by a barometer at that level. To avoid risks of air leaking through this case were it to be pierced by a moving axle, this clock is actuated by weights lifted electrically by a small primary battery. The slight electric current required has no perturbing effect on the clock. This time-piece, provided with an escapement of great excellence, was manufactured by Clemens Riefler of Munich.
At the Observatory of the Case School of Applied Science, Cleveland, Ohio, another Riefler clock has a mean error of but .015 second per day. This means that in a year the total error is not more than 5.475 seconds, or one part in 5,760,000 of the 365 days. Such errors, minute as they are, give a good deal of trouble when they are irregular, that is, when the clock is sometimes slow, sometimes fast, in a fashion apparently lawless. When the divergences are fairly constant they can usually be traced to their source, making it feasible to apply a remedy.