We see now the necessity for correcting the standard clock by gradually increasing or decreasing the rate, for if it were done rapidly, the controlled clocks would break away from the control, and not be slowed and accelerated with the standard. At Greenwich the correction, usually only a fraction of a second, is made a little before the hours of 10 A.M. and 1 P.M., since at those instants a distribution of time is made throughout the country. This distribution is made as follows:—
An electric circuit is broken in two places at the standard clock, one place of which is connected for some seconds on either side of each hour, while the other is connected at each sixtieth second; both breaks can therefore be only connected at the commencement of each hour, and then only can the current pass. We will call this, therefore, the hourly current: it acts on the magnet discharging the Greenwich time ball at one o’clock daily, and on the magnet of the hourly relay shown in Fig. 129, which completes various circuits. One goes to the London Bridge station of the South-Eastern Railway Co., and the other to the General Post Office for further distribution. The bell and galvanometer in the figure marked “S. E. R. hourly signal and Deal ball,” and “Post Office Telegraphs” show the passage of these signals. We have now got the hourly signal at the Post Office, and this is distributed by means of the Chronopher, or rather Chronophers, for there are two, the old one originally constructed by Mr. Yarley, and brought from Telegraph Street on the removal to St. Martin’s-le-Grand, and a new one, much larger, shown in the accompanying Fig. 130. It is to this that the Greenwich wire is led, and the current transmitted to the different lines. The lines are divided into four groups, (1) the metropolitan, (2) short provincial, (3) medium provincial, (4) long provincial; the first being wires passing to points in London only, the second to places within about 50 miles of London, the third to more distant places, and the fourth to the more distant places still, requiring signals. The ends of each of the four groups are brought together, and each group has its separate relay. These four relays—the left-hand four shown in Fig. 130—are all acted upon by the Greenwich signal and therefore act simultaneously, each relay sending a portion of the current of its battery through each wire of its group.