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.
Fig. 130.—The Chronopher.
The metropolitan group, being used only for time purposes, is always connected with the relay, but to the country, signals are sent only twice a day, namely, at 10 A.M. and at 1 P.M., and as the ordinary wires are used for this purpose, they must be switched into communication with Nos. 2, 3, and 4 relays. The action at each hour is as follows:—The wires leading to the respective towns are connected with their speaking instruments through a contact spring; these contact springs are shown in the figure in a row, like the keys of a piano; along the keys runs a flat bar which at a short time before 10 A.M. and 1 P.M. is turned on its axis by the clockwork above, by so doing it presses back all the keys from their respective studs, and so cuts off communication with the speaking instruments, and puts the wires into communication with the bar, which is divided into three insulated portions, each in communication with a relay and battery; the batteries and relays become connected with their respective groups, and a constant current flows through all the wires to the distant stations serving as a warning. When the Greenwich current arrives the relays reverse the currents, and this gives the exact time. Shortly afterwards the clock turns back the rod and the springs go into contact with their respective instruments, and all goes on as before. One of the remaining relays of the apparatus sends a current to Westminster clock tower for the rating of the clock there, but it is in no way mechanically governed by the current. The apparatus is entirely automatic, and to judge of the degree of accuracy obtained an experiment was made. One of the distributing wires was connected with a return wire to Greenwich, and the outgoing current to the Post Office and the incoming one were passed round galvanometers, when no sensible difference could be seen in the indications.
At 10 A.M. a considerable distribution goes on by hand. At this instant a sound signal is heard from the chronopher, and the clerks immediately transmit signals through the ordinary instruments to some 600 places; these again act as centres distributing the time to railway stations and smaller places.
The methods of signalling the time are various; at some places, as at Edinburgh, Newcastle, Sunderland, Dundee, Middlesborough, and Kendal, a gun is fired at 1 P.M. The history of the introduction of time-guns is a somewhat curious one.
In August 1863, during the meeting of the British Association at Newcastle, Mr. N. J. Holmes contrived the first electric time-gun. This gun was fired by the electric current direct from the Royal Observatory at Edinburgh, 120 miles distant. Time-guns were afterwards experimentally fired at North Shields and Sunderland; the Sunderland gun was after a time withdrawn; the Newcastle and North Shields time-guns are regularly fired every day at 1 P.M. Four time-guns were mounted in Glasgow, also to be fired by the electric current from Edinburgh; a large 32-pounder was placed at Port Dundas, on the banks of the Forth and Clyde Canal; a second small gun was placed near the Royal Exchange; a third 18-pounder at the Bromielaw, for the benefit of Clyde ships in harbour; and a fourth twenty-five miles further down the Clyde, at the Albert Quay, Greenock, for the vessels anchored off the tail of the bank. These four guns, and the two at Newcastle, were regularly fired from the Royal Observatory, Edinburgh, for some weeks. A local jealousy springing up amongst a few of the Glasgow College Professors and the Edinburgh Observatory, against the introduction of mean-time into Glasgow from the Royal Observatory Edinburgh, instead of deriving it from the Glasgow Observatory clock (the longitude of which was undetermined at that time), the originator of the guns, Mr. Holmes, was cited before the police-court, charged under the Act with discharging firearms in the public streets. The jealousy ended in the withdrawal of the guns, and Glasgow, from then until now, has been without any practical register of true time.[[15]]
Another system of time signalling is to expose a ball to view on the top of a building, and drop it, as in the case of the ball automatically dropped at Greenwich every day. We have already mentioned that one of the wires from the Greenwich Observatory connects it with the London Bridge Station, and this is used for dropping the time-ball at Deal. In return for the hourly signals the Company give up the use of the wires to Deal for two or three minutes about 1 P.M., when the Deal wire is switched into communication with the Greenwich wire by a clock, just in the same manner as at the Post Office, and communication is also made at Ashford and Deal, in order that the current shall go to the time-ball. In order that they shall know at Greenwich that the ball has fallen correctly, arrangements are made so that the ball on falling sends a return current back to Greenwich. It appears that erroneous drops are rare, but, if such is the case, a black flag is immediately hoisted and the ball dropped at 2 P.M.
Hourly signals are distributed on the metropolitan lines and to the “British Horological Institute” for Clerkenwell; the leading London chronometer makers also receive them privately.
We now come to deal with one of the practical uses of the clock and transit instrument with reference to determining longitudes.