Popular Science Monthly/Volume 22/December 1882/Time-Keeping in London I
|TIME-KEEPING IN LONDON.|
IT is proposed in this paper to describe some special features of the instruments by which time is kept at the Royal Observatory, Greenwich, the means for correcting them, and the methods and instruments by which time-signals are distributed from the observatory to London and elsewhere.
The primary standard time-keeper of England is a sidereal clock kept in the basement of the Royal Observatory, Greenwich. This clock is of the best construction, and is, moreover, provided with the most approved apparatus for compensation and correction.
Experience has shown that the best results are obtained when the connection between the driving-weight and the pendulum of a clock is as slight as possible. This has been accomplished in the Greenwich clock by the use of an elegant escapement, the details of which are shown in Fig. 1, representing a back view of the clock-train. The crutch-axis, supported by the arm (c) and the back plate (b) of the clock-train, carries an arm (e), attached at f to the left-hand pallet arm. The pallets are carried by the crutch-rod (d). At gis attached a detent projecting toward the left and ending in a light curved
spring. Near the top of the escape-wheel this detent carries a jeweled pin which locks the wheel. The action is as follows: When the pendulum swings toward the left, the arm (e) lifts the delicate spring at the end of the detent, the wheel is released and drops forward so that a tooth presses against the face of the pallet and gives an pulse to the pendulum; the spring at the end of the detent immediately locks the wheel again, and the pendulum swings on freely to the left. When the pendulum swings to the right, the light spring at the end of the detent lets it pass without unlocking the wheel. The right-hand pallet is only intended to catch the wheel in case of accident and forms no essential part of the escapement. Thus, it will be seen, the pendulum is quite free except during a part of every alternate second, when it releases the escapement and receives an impulse; the seconds-hand, attached to the escape-wheel, moves only once every two seconds.
The most important source of error in the running of a fine clock is the change in the length of the pendulum due to change of temperature. Two methods suggest themselves of eliminating this error: 1. To put the clock where it will not be subject to changes of temperature. 2. To counteract the effect of changes of temperature. To this end various kinds of pendulums have been devised, notably the mercurial and gridiron forms, which are known under the general name of "compensating pendulums." At Greenwich the two methods are combined to insure complete success. The clock is placed in the magnetic basement of the observatory, where the temperature is as nearly uniform as possible, and apparatus is provided to annul the effect of any change of temperature which might occur.
Tests made with a mercurial pendulum disclosed the fact that the steel rod responded more quickly than the mercury to a change of temperature, and that consequently an appreciable interval of time was required for perfect compensation; a modification of the gridiron form, shown in Fig. 2, was therefore adopted. The pendulum was designed by Messrs. E. Dent & Co., of London, for the Transit of Venus Expedition (1874), but has since been used for the primary standard time-keeper of the United Kingdom. Its construction will be best understood by reference to the section shown in Fig. 3. To the lower end of a steel rod, suspended in the ordinary manner, is attached the screw for rating the pendulum. On this screw and surrounding the rod rests a zinc tube, extending upward; inclosing the zinc tube and attached to its top is a steel tube extending downward; on a collar, at the lower end of the steel tube, hangs the cylindrical leaden bob, attached at its center. Slots and holes are cut in the tubes in order to equally expose all parts. The following table, taken from the official records of the Royal Observatory, is published by Messrs. E. Dent & Co., for the purpose of showing the performance of a clock with steel and zinc pendulum:
|DATE||Clock slow of Green-
wich sidereal time.
|1871—September||3||21||14||31·8||. .||. .|
During the whole time of rating, the clock was situated in a small hut erected for observing the Transit of Venus. No record of the temperature of the hut was kept, but the variations would be very
similar to those of the external air, whose average temperature for each interval is given in the table.
The compensating action of the pendulum evidently depends upon the relative lengths of steel and zinc, and it is easily possible that great difficulty would be experienced in cutting and fitting tubes of exactly the right length; to complete the adjustment a very delicate contrivance is added.
Two compound bars of brass and steel (h and i, Fig. 1), with small weights at their ends, are hung to the crutch-axis by means of a collar loose enough to be easily turned. The rods are so made that under normal conditions the brass and steel are of the same length, and the two bars are in the same straight line; the center of gravity of the rods and the weights (regarded as one body) is therefore in the axis, and the weights are balanced in every position, no matter what angle the line of the rods makes with the plane of the horizon; they affect the pendulum only by their inertia. But, when a change in temperature occurs, the brass and steel become of unequal length, owing to a difference in the co-efficients of expansion of the two metals, the rods are bent, and the center of gravity of the rods and weights is no longer in the axis, nor is it in the same vertical plane as the axis except when the weights are in a horizontal line; so that an unbalanced force is introduced whose compensating action varies from a maximum when the weights are in a horizontal line, to zero when the weights "are in a vertical line. To be explicit, suppose the rods to be horizontal and the brass uppermost, and let there be an increase of temperature. The brass will expand more than the steel, and, the rods being bent downward, the weights will be lowered. As the pendulum swings the weights swing with it, and are continually trying to get back to a horizontal position where they would balance each other; if they were swinging alone, they would evidently swing faster than the pendulum, and therefore, being attached, they accelerate its motion. If the steel were uppermost, the weights would be raised with an increase of temperature and the pendulum retarded. If the rods were both vertical, a change of temperature would only throw the center of gravity of the two weights to one side or the other of the axis, but would not raise or lower it; this would only introduce a continuous force tending to make the pendulum oscillate farther on one side than the other, but not affecting its rate. At intermediate positions between the vertical and horizontal, the change in the position of the center of gravity due to a change of temperature would vary with the angle made by the line joining the centers of gravity of the two weights with the plane of the horizon; any required compensating action, between the limits above mentioned, for a known change of temperature, can therefore be obtained by setting the rods at the proper angle.
In order to make a small change in the rate without stopping the pendulum, the device shown in Fig. 1 has been employed: A weight (k) slides freely on the crutch-rod shown back of it in the figure, but is held by the screw on the end of the spindle (l) which bangs from the nut (m) at the crutch-axis. By turning the nut (m) the weight (k) can be lowered or raised, and this makes the clock gain or lose.
But the nicety of the correction of variations due to changes of temperature has brought to light variations due to another cause commonly quite overlooked; it has been found that the pendulum is affected by changes of barometric pressure. A change in the barometer of an inch and a half will sensibly alter the rate of the pendulum. The difficulty might be avoided by placing the clock in a vacuum, but this is evidently impracticable. In the Greenwich clock the method shown in Fig. 4 has been adopted to counteract the effects
of barometric changes. To the pendulum-bob are attached two vertical bar-magnets, one in front (a) with the north pole down, the other at the back (and therefore not shown in the figure), with the south pole down. Below these and normally at a distance of 33⁄4 inches from them is a horseshoe magnet (b) which hangs on one end of a lever (c) nicely balanced on knife-edges at A; the other end of the lever (c) rests by means of a rod (d) on a float (e) in the shorter leg of a siphon 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, 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 hollow galvanic coil, called "the accelerating or retarding coil," fastened to the clock-case and operated by a special battery. The attraction or repulsion, between the magnet and the coil, produced by sending currents in opposite directions, gives any required acceleration or
retardation to the pendulum. Care must, of course, be taken that the correction be not made too quickly, else the clock, instead of being controlled by the current, will break away from control, and the error will be increased. It is now so arranged that the current will produce 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 a. m. and 1 p. m., 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.
This hourly current acts on the magnet which drops the Greenwich time-hall daily at one o'clock, and on the magnet of the hourly relay (to the left in Fig. 5) which completes several independent circuits, each controlling a separate line of wire. One of these extends to the central telegraph station at the General Post-Office in London, and another to the London Bridge Station of the Southeastern Railway. The hell and galvanometer marked in Fig. 5 "P. O. Telegraphs" and "S. E. R. Hourly Signal and Deal Ball" show the passage of these currents.
Thus far the service is under the control of the astronomer royal, and he holds himself responsible to send the signals described along each line every hour of the day and night with the greatest attainable accuracy. The signals are generally correct within one tenth of a second of error. Should, however, by any accident, an hourly signal be in error, even to half a second, another signal is immediately sent, announcing that the last was not reliable. Special pains are then taken that the next hourly signal be correct. Here the responsibility of the astronomer royal (except for the dropping of the Deal ball, to be explained later) ends.
On the other hand, it is to be remarked that the Post-Office Department, which undertakes the distribution of these signals to London and the country, agrees to furnish subscribers, not with correct signals, but with the signals which they receive from Greenwich. The Greenwich signals, however, being considered everywhere in England as absolutely correct, constitute a standard from which there is no appeal.
[To be continued.]
- Figs. 1, 3, 4, 5, and 6, have been taken from Lockyer's "Stargazing," through the courtesy of Macmillan & Co., London, publishers, by permission of the author.
- 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.
- For an illustration of the Jones system for regulating clocks at a distance, see article on "Time-keeping in Paris," "Popular Science Monthly," January, 1882.