Page:Popular Science Monthly Volume 22.djvu/176

164 a correction of one second in about ten seconds. The correction is made as follows: Between the sidereal chronometer (b, Fig. 5) and the mean-time chronometer (c) there is a commutator (d). By moving its handle toward the right, a current is sent through the "accelerating or retarding coil" which accelerates the mean solar standard; by moving the handle toward the left, the current goes through the coil in the opposite direction, and retards the mean solar standard; in the intermediate position (shown in the figure) no action takes place. The operator, having ascertained the error of the sidereal standard and its sympathetic chronometer, by astronomical observation as described, applies this error to the face-reading of the sidereal chronometer, and gets the exact sidereal time; by simple reduction he finds the corresponding mean solar time, and, by comparison, the error of the meantime chronometer; he then moves the handle of the commutator, and corrects the error of the mean solar standard, and of all the clocks controlled by it, without leaving his position in the computing-room. This correction can be made at any instant when the exact time is desired; it is usually made at 10 and 1, because at those hours a general distribution of time-signals takes place.

The mean solar standard serves for the distribution of accurate time in the following ways:

Nearly all the mean-time clocks in the Royal Observatory are driven by the standard clock; they are, in fact, simply dials whose hands are moved in the same way and by the same battery as the hands of the standard itself. These clocks are placed in the various rooms of the observatory, so that the astronomers have the exact time close to any of their instruments. One of them is in the wall surrounding the grounds, and will be familiar to every one who has visited the observatory; several are placed in the chronometer-room, where the navy and other chronometers are corrected and regulated.

The seconds-relay (a, Fig. 5), already referred to, is also driven by the mean solar standard.

Until 1880 the standard clock controlled, by seconds-beats, a number of clocks on a private wire in London, which were made to beat synchronously with the standard by an application of the Jones system, in which the electric current is used, not as a driver, but as a regulator of clocks already running with small error and by means of their own motive powers. This plan, though still used within the observatory, has been abandoned in London.

With the standard clock is connected another electric circuit, open in two places. These are both automatically closed by the clock, one at the end of each minute, but the other only for some seconds on either side of the end of each hour; so that they are both closed only at the end of each hour, and then only can the current pass.