Page:Popular Science Monthly Volume 22.djvu/174

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barometer. Counterpoises are added at f to balance the magnet (b). A plan of the lever on a smaller scale and a section at A are also shown in the figure. The barometer-tube is made so much larger in the shorter than in the longer leg that a change of one inch in the barometer would move the float in the shorter leg only two tenths of an inch. A rise or fall in the barometer causes a corresponding motion in the horseshoe magnet, and thus varies the intensity of its attraction for the magnets on the pendulum-bob. By proper adjustment this varying attraction is made to furnish the required compensation.

The small error which remains, notwithstanding the above-detailed provisions for correction, is allowed to accumulate, but is determined daily (unless clouds prevent) by transit observations,[1] so that the exact sidereal time is always known.

The standard sidereal clock registers its beats upon the chronograph record; controls, by electric connection, all the sidereal clocks in the different rooms of the observatory; and drives a sidereal chronometer (b, Fig. 5), in agreement with itself, in the computing and time-distributing room.

The secondary regulator of the time of England is the mean solar standard clock at the Royal Observatory, which was specially erected in 1852 for service in the time-signal system, of which it is now the most important instrument. This clock has a seconds-pendulum, which closes an electric circuit as it swings to the right. An electromagnet in the circuit lifts a small weight, which is discharged upon the pendulum as it swings to the left, and gives it an impulse; this being repeated at each vibration is sufficient to keep it in motion. The pendulum also closes other galvanic circuits—one as it swings to the right, another as it swings to the left—which send currents alternately positive and negative through electro-magnets, alternately attracting and repelling bar-magnets fastened to an axis, which thus receives a reciprocating motion. An arm projecting from this axis moves the seconds-wheel one tooth forward each second; proper gearing gives motion to the minute and hour wheels.

The mean solar standard, besides controlling other clocks, to be enumerated later, drives a seconds-relay (a, Fig. 5), which controls a mean-time chronometer (c).

All the clocks controlled by the mean solar standard are required to indicate exact Greenwich local time; the error can not therefore be allowed to accumulate, and the means of correction are provided. Carried by an arm projecting from the pendulum-rod of the mean solar standard is a magnet, five inches long, which swings just over a

  1. The difference between the clock-time of the transit of a star over the meridian (corrected for errors of position of the instrument, and for "personal equation") and the right ascension of the star for the day, taken from the nautical almanac, is the error of the clock.