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Mercurial Compensation

In the mercurial pendulum with a glass jar, the mercury does not answer so quickly to a change of temperature as the steel rod, and preference is therefore now generally given to thin metal jars; still the elegant appearance of the glass jar in a stirrup renders it suitable for show regulators, for which it is still retained. The following are the dimensions of a good seconds pendulum of this class: Steel rod, .3 inch in diameter, 43 inches long from top of free part of suspension spring to bottom of sole of stirrup; side rods of stirrup, .3 inch wide and .125 inch thick; height of stirrup inside, 8 inches, bottom of stirrup, .5 inch thick with a recess turned out to receive the jar; glass jar 7.6 inches deep and 2 inches in diameter inside, outside 2.25 in diameter, and 7.8 inches high; height of mercury in the jar about 7.4 inches; the weight of mercury 11 lbs. 12 oz. The steel parts may with advantage be annealed to guard against the possibility of magnetism.

The mercury divided between two jars answers quicker to changes of temperature.

Precision clocks with mercurial pendulums have jars larger in diameter than two inches, made of cast iron enameled on the inside, or of steel. Great care should be taken, when filling the mercury jar, to avoid air bubbles. The best plan is, push the center of a good silk handkerchief into the jar and pour in the mercury through a long boxwood or other funnel with but a mere pinhole for the outlet. When the whole of the mercury is poured in, carefully draw up the handkerchief by its four corners. The jar of mercury, with a piece of bladder tied over the top, may then be subjected to a temperature of about 120 deg. for a week or two.

It is important to get the mercury as pure as possible for a pendulum. A good way of removing impurities is to add sulphuric acid to the mercury and shake the mixture well. The metal is then washed, and afterwards dried on blotting paper. Another method of purifying mercury is to put it in a bottle with a little finely-powdered loaf sugar. The bottle is stoppered and shaken for a few minutes, then opened and fresh air blown in with a pair of bellows. After this operation has been repeated three or four times, the mercury may be filtered by pouring it into a cone of smooth writing-paper, the apex of which has been pierced with a fine pin. The sugar and impurities will be retained by the cone. Some filter mercury by squeezing it through a piece of fine chamois leather. In dealing with mercury, care should be taken to avoid the injurious vapor which rises from it even at the ordinary temperature of the air, and of course more freely at higher temperatures.