Popular Science Monthly/Volume 22/January 1883/Time-Keeping in London II
THE distribution of the Greenwich signals from the General Post-Office in London is effected by means of the Chronopher or Time-carrier, shown in perspective in Fig. 6, and in front elevation in Fig. 7. To this instrument the hourly signal from the observatory is sent by means of a special under-ground wire. Branching out from it are four groups of wires: 1. Metropolitan, running to points in London only. 2. Provincial Short, to points not more than fifty miles from London, as Brighton, etc. 3. Provincial Medium, to points farther away, as Hull, etc. 4. Provincial Long, to extreme points, as Edinburgh, Belfast, etc. The ends of each of the four groups are brought
together, and each group has a separate relay, in order that the shorter may not unduly deprive the longer lines of their share of the current. The four relays are all worked by the hourly signal from Greenwich, and therefore act simultaneously. The lines of the Metropolitan group are used only for time purposes, and are therefore always connected with their relay, and distribute the signals hourly. But the lines of the other groups are in use generally for ordinary telegraphic purposes, and distribute time-signals only at ten and one o'clock. At these hours, therefore, the wires must be switched off from their ordinary duty, and placed in communication with their respective relays to be ready to receive the time-signals. The electrical working of the apparatus which accomplishes this will be understood by reference to Fig. 8. Under normal conditions, the current from the observatory passes
directly through the chronopher, and out at the galvanometer G', to the tower-clock at the Houses of Parliament, "Westminster. This clock has a gravity escapement, and a metallic compensating pendulum, very similar to the pendulum of the Sidereal Standard, already described, and runs with a rate of less than one second per week. The
current from Greenwich in no way controls the Westminster clock, hut is simply used for rating the clock hy comparison ; when change of rate is necessary, weights are added or removed at the pendulum. Each of the line wires is in permanent connection with one of a set of jointed vertical bars (B, B', B"), which, except at the times for the signal, are kept in contact by springs with cocks in the circuit of the wire; but at the times for the signal a long metallic bar (C) acting as a cam (better shown in Fig. 7, in front of the vertical bars), is made by clock-work to disconnect all these bars from their instruments. The bar (C) is divided into three parts, corresponding to the long, medium, and short provincial lines, insulated from each other, and connected respectively with the bars of the relays (V, V', V") through the galvanometers (g, g', g", Fig. 8). The left or rest contacts of these relays are in connection with the zinc poles of separate batteries, whose copper poles are grounded, so that, when the bars of these relays are put in connection with the line wires, a zinc or "preliminary" current is ready to be sent out; this current prevents the distant relays from being actuated by contacts or accidental currents, and serves as a warning signal. The right-hand contacts of the relays are connected respectively with the copper poles of separate batteries whose zinc poles are grounded, so that, when the bars are moved over to the right (which is done by the incoming Greenwich current), the outgoing current is reversed, and this constitutes the signal. The relay V" is for distributing the signals only to points in the metropolis, and, as the wires on these lines are under ground, no "preliminary" current is necessary.
The mechanical operation of the apparatus is as follows: On the clock (R, Fig. 9) there is an ebonite wheel (W) in which are two notches (N, N') corresponding to 10 a. m. and 1 p. m. Shortly before 10 a. m. the pin (P) on one arm of the forked lever (L) falls into the notch (N), allowing the end (Q) of the other arm to rest on the ebonite hour wheel (T). About two minutes before the hour, the end (Q) comes against the contact (S), and completes the circuit of the local battery (IT, Fig. 8) through the starting magnet (M, Fig. 9) and sets the clock-train (shown in Fig. 7) in motion, pressing the cam (C) against the vertical bars, disconnecting them from their instruments, and connecting them respectively, in groups as already shown, with the relays (V, V, V"), in readiness to send a "preliminary" current to the line wires. At ten seconds to the hour an insulated pin (i, Fig. 9) on the wheel (T) lifts the lower arm of the forked lever (F),so that its upper arm comes in contact with a small cam on the arbor of the escape wheel (K). This contact closes the circuit of the battery (U) through the coils of the two relays (Z, Z'). The relay (Z) puts on the earth connection at (E), for the four relays (V, V,' V", V'"), so that the current from Greenwich may be received and divided between them, while the relay (Z') disconnects the Westminster clock-wire and connects it with the metropolitan lines to receive the signal from the relay (V"). The relays (V, V, V", V'") have a resistance of 5,000 ohms to allow of the splitting of the current. At precisely ten o'clock the Greenwich signal reverses the current on the lines, and thus gives the exact time. At ten seconds past the hour the contact between H and K is broken, the relay-bars go back to their normal position, the train-work moves away the cam (C), and restores the vertical bars to connection with their instruments.
The apparatus which effects the shunting at one o'clock is somewhat different in construction. The pin (P, Fig. 9) falls into the notch (N'),
a pin (p) on the wheel (W) coming against the arm (I) of the forked lever (Y) raises the flexible arm (G) against the upper contact (D), so that the circuit of the local battery (U) is closed through the starting magnet (M), which operates the one o'clock train-work.
Wires which receive both the ten and one o'clock signals pass through both switching arrangements.
For the hourly currents on metropolitan lines the relay (V''') serves, by closing the circuit of the battery (U) at the contact (K), the rest of the apparatus remaining inoperative.
The actual interval during which the Greenwich as well as the provincial wires on which the time-signal is distributed are kept in circuit being only twenty seconds, the chance of interruption from contact currents is reduced to a minimum.
The batteries in use are large Leclanché cells, and the power is distributed as follows:
|Copper or "time"
|Zinc or "preliminary"
|Metropolitan lines||40||"||. .||"|
The Greenwich signal, thus distributed by the chronopher, goes to all parts of the kingdom, and affects receiving instruments provided for the purpose. These are of various kinds; ordinary telegraphic sounders, electric bells, and galvanometers have been used with success to note the arrival of the signal. The current has also been made to drop time-balls on the tops of buildings, to expose a model time ball to view, and to fire guns.
To test the accuracy of the signals, experiment has been made by returning a wire to Greenwich from the chronopher, and comparing the signal received on this wire with the signal sent from the observatory; no difference could be perceived between the indications of two galvanometers placed side by side showing the passage of both currents. The signals were thus shown to be entirely reliable. But it does not seem likely that the chronopher will be introduced elsewhere, because simpler means have been devised for splitting up the current and distributing the signals.
The whole system is under the control of the Post-Office Department. They own the wires—which, except in London, are the ordinary telegraph-wires and therefore contract to keep them in order, to clear them each day at the signal-times, and to deliver at these times the Greenwich signal. Maintenance of lines and apparatus not the property of the department is undertaken by the department for any period not less than one year at specified rates. A simple form of agreement has been prepared, which every renter is required to sign. This agreement, as a rule, is for not less than three years, and is terminable at three months' notice given previous to the end of the fixed term, or, failing such notice, on payment of such sum as the department may accept instead. But where the expense of construction is considerable, the term must not be less than from five to seven years, the latter period being stipulated when the proposed line is in an outlying district and would be specially provided for a single renter, and when it is not probable that there would be other renters.
The annual charges for the use of wires and apparatus are as follows:
From London to the country: For the 10 a. m. signal, 12 to £17 = $60 to $85. For the 1 p. m. signal, £27 to £32 = $135 to $160. In London: For the hourly signal within a radius of two miles from the General Post-Office, £15 = $75. But if the person desiring the signal is off the line of the telegraph, he must pay, besides a stipulated rental, an additional sum for the use of the wire which the department is compelled to put up specially for him. The rental is in all cases payable yearly in advance.
In 1880 there were one hundred subscribers to the system, of whom nineteen were in London, and eighty-one scattered through England, with a few in Scotland and Ireland.
Besides this general automatic distribution of the time-signals, a considerable distribution of the 10 a. m. signal goes on by hand. At that instant the chronopher makes a sound which an operator sits ready to catch by ear. Upon hearing it he immediately dispatches a signal by the ordinary telegraphic instrument, and this signal is received at six hundred or more places, which again serve as distributing points for more distant places. These are usually railway or post offices in towns not supplied by the chronopher, which by virtue of authority become the regulators of the clocks of the surrounding district.
The wire from the observatory to London Bridge carries signals hourly from the mean solar standard to a clock at the station of the Southeastern Railway, which by changing connections sends Greenwich time to different stations along the line as may be required. For this service the Southeastern Railway gives the observatory the use of its wire daily, for a few minutes, at 1 p. m. At this time the current from the observatory drops the time-ball at Deal, which was erected in 1855, to give time to the shipping in the Downs, and is the only official coast time-signal. The ball in falling sends a "return" signal to the observatory. The record shows that about once in two months high wind prevents the raising of the ball, about once in six weeks it fails to fall on account of some fault in the electric connections, and about once a year it drops out of time. Under such circumstances it is dropped correctly at 2 p. m.
By special arrangement with the observatory a few London jewelers receive the hourly Greenwich current on private wires. This they use for the correction of their own time-keepers and in some cases for distribution. Prominent among these are the Messrs. Barraud & Lund, of Cornhill, who have patented a method for the synchronization of clocks. Their plan is put forward as a simple and effectual means of setting any number of ordinary clocks to the same standard time. All attempts to control clocks have been set aside as impracticable, and a system adopted whereby the clock is automatically "set to time" every hour, or at such intervals as may be arranged. The apparatus can here be described only in brief. There are three essential parts, the standard clock, the distributor, and the synchronizer.
The standard clock is an astronomical regulator with mercurial pendulum and dead-beat escapement, and closes an electric circuit at the sixtieth second of each hour. Another regulator, technically called "Lobby," is for use in case of accident to "Standard." They are so connected that a single failure of "Standard" to send out a signal at the proper time brings "Lobby" into action for the next signal, and, in order that "Lobby" may always be ready for service, an intentional breakdown of "Standard" occurs automatically at eight each morning, and the nine o'clock signal is sent out by "Lobby"; which of the two is in operation is shown by indicators connected with the clocks (Fig. 10). Should a breakdown occur, the indicator of "Standard" would show missed, and that of "Lobby" at work.
The error of the standard clock is determined daily by comparison with the Greenwich signal. An ordinary dotting chronograph is set to the standard clock, and the Greenwich signal makes a dot on the chronograph-dial which gives at once the error of the standard and can be read off at leisure. It is corrected by electric means. The pendulum carries a small permanent magnet which swings over a resistance-coil about 16 inch distant. The coil is connected with the commutator in the test-box (Fig. 11), consisting of a clock commutator with plugs for "Standard" and "Lobby," a current commutator with plugs for "Fast" and "Slow," and a small time-piece, shown at the top. The time-piece has only a minute-hand, and is made so as to stop itself and break circuit at XII, but closes circuit when running. The working is thus: Suppose "Standard" is found to be slow. Plugs are inserted for "Standard" and "Slow," and the hand of the time-piece is set back a required number of minutes. It then runs to XII and stops. In this interval the action between the magnet and the coil has exactly corrected the standard clock. For every 10 second of error the hand of the time-piece must be set back five minutes. When the setting is done, no further attention is required, all else being automatic.
The distributor (shown in Fig. 11) consists of twelve contact-springs, each connected with a line of wire running through a district of London, and twelve contact-screws, each connected with a battery.
The springs converging to the center press up against a small plate, one inch in diameter, which is controlled by an electro-magnet in the circuit of the current which the standard clock sends out hourly. When the signal comes, the plate is pulled down and presses every spring against its contact-screw, and the signal goes out over each of the lines.
The synchronizer is the receiver of the signal, and consists essentially of an electro-magnet, in the circuit of one or other of the lines from the distributor, with armature carrying two counterpoised levers
|Fig. 13.—Face of Clock with
Other ingenious arrangements have been added to guard against danger, always present to long lines of wire, and for testing the condition of the lines, but a description of them can not here be given.
The advantages claimed for the system are:
1. That any number of clocks of any varying sizes can be synchronized to any agreed standard time-keeper.
2. That the mechanism is, when not in momentary use, entirely detached from the works of the clock.
3. That it can be applied to existing clocks.
4. That any failure in the transmission of the time-current leaves the clock going in the ordinary way, to be "set to time" by the next completed current.
5. That the clocks are kept to time whether having otherwise either a gaining or losing rate, even if such rate amounts to many minutes a day.
In London the system has been in successful operation for about five years, and has been used over a wire four hundred miles long. The subscribers number about five hundred, among them many rail-roads and public institutions.
In connection with the synchronized clocks, Messrs. Barraud and Lund have also established time-bells and flashing-signals, which afford
time-signals both to the ear and eye. These are shown in Figs. 15 and 16. The bell is an ordinary electric bell, and is rung by the regulating clock, which closes the circuit at the instant the signal is desired. The flashing-signal consists of a red vertical disk on a vertical axis, which normally shows only its edge, but is made to revolve once on its axis in four sudden jumps, by simple mechanism in connection with electro-magnets, when the regulator, by closing the circuit, sends the current. The appearance is that of two flashes of red as the disk revolves.
In many places where noise prevents hearing a bell, the flashing-signal becomes a necessity. It is in use at the London Stock change, and serves to indicate the exact instant of noon.
The method of synchronizing clocks is becoming rapidly popular throughout the world, and has been patented in most civilized countries. It is already in use in Australia and South America, and in some of the countries on the Continent of Europe. In this country, at New Haven, Connecticut, a "Standard Time Company" Las been formed, who have bought the patent for the whole of the American Continent, and are now engaged in manufacturing synchronizers. An effort will be made by them to bring about a concerted system of time-signaling throughout the country. Local affiliated companies will be formed, and there is little doubt that the great simplicity and practical success of the method, combined with its cheapness, will secure its extensive adoption in all the large cities of the country.
- There are actually two of these; the one shown in the figure is the new and larger one.
- For a description of the chronopher, from which the above is condensed, and for drawings from which Figs. 7, 8, and 9 have been made, the writer is indebted to William H. Preece, Esq., Superintendent of Telegraphs, London.
- The Greenwich signals are sent into Ireland only for purposes of comparison; Dublin time is used throughout the island.
- The small figures 1, 2, 3, 4, to the right of Fig. 8 and to the left of Fig. 9, show the connection between the wires.
- Difference in charge for the same signal depends on the length of wire which the department is compelled to put up specially for the subscriber. The one o'clock signal is more expensive, because the wires are busier with telegraph duties at that hour than at 10 a. m.
- Figs. 10, 11, 12, and 13, hare been reduced from drawings in "The Railway Engineer," London, by permission of Messrs. Barraud & Lund.