DISTRIBUTION OF TIME SIGNALS | ||||||||
| P.O. ENGINEERING DEPARTMENT TECHNICAL INSTRUCTIONS XI CLOCKS AND TIME DISTRIBUTION 1927 SECTION V
112. Distribution of Greenwich Mean Time Signals
For security of service, a second underground wire on a different route is always immediately available between Greenwich Observatory and the Central Telegraph Office (CTO). A schematic representation of the Time Distribution System is shown in Figure 44 or an earlier Diagram GMT6 and a diagram showing the distribution of Greenwich Mean Time to various centres in the British Isles is given in Appendix D. 113. Control Clock for Main Chronopher At two minutes to the hour a contact is made and a battery brought into action which operates the relays, thus disconnecting the time-wires from their working sets and preparing the way for the passage of the Time signal at the. hour. This battery current is known as the "Preliminary" and its object is to warn time various offices and renters to prepare for the actual Time signal. Exactly at the hour, Greenwich Observatory sends the Time signal in the form of a current, the duration of which is one second in the reverse direction to that of the "Preliminary" signal of the control clock. When the Time signal has passed, the preliminary current; restored for a further period of 2 minutes 8 seconds. At 1.00 p.m., however, an additional operation takes place after the Time signal has passed. At certain Admiralty Stations (with time balls), viz:- Deal, Portsmouth, Portland and Plymouth, switching arrangements are introduced to enable these stations to send a return signal or acknowledgment to Greenwich (Figure 46 or diagram GMT10) for the purpose of enabling the Observatory to check the performance of the clocks at the stations. In order that this may be done, the preliminary current is out off at 5 seconds after 1 p.m. leaving the line clear for the transmission of the return signals. The times at which these return signals are transmitted:- At 1h 1m 20s the preliminary current is restored until 1h
0m 8s, when the lines are again transferred to their working sets.
Greenwich Mean Time - Return Signalling Connections 114. The Chronopher
Chronopher, GPO West / Central Telegraph Office The Chronopher control clock, which has a slight gaining rate, operates the Relays 90A (R1) preparatory to the passage of the Greenwich Mean Time signal, thus bunching the lines over which the signal has to be transmitted and sending through them a current from the positive pole of an 80 volt battery via the armatures of the transmitting relaying sounders R.S. This positive current gives the preliminary or warning signal. At exactly 10.00 a.m. the G.M.T. signal, passing through this non-polarized relay R2, moves the tongue to the left-hand contact, thereby putting "Earth" on the electromagnet circuits of the relaying sounders, actuating their armatures and causing a reversal of the current on all the lines. In addition, the synchronization of the control clock is effected. On the cessation of the current the Relays 90A are de-energized and the lines restored to their working sets. In the case of Admiralty circuits, provision is made for the transmission through R3 of return acknowledgment signals to Greenwich at the times shown in Figure 48 or diagram GMT1, which also gives the times at which the control clock operates daily. The "Hourly" signal is transmitted from, the control clock to Metropolitan hourly renters, to various parts of the Central Telegraph Office and to the Engineer-in-Chief's Office. The signal operates time bells or gongs. The galvanometers G are introduced to show that the currents are being correctly sent out, and the indicators I to show that the Admiralty return signals are passing through to Greenwich. Suitable fuses F are placed in the circuits. The local, circuit of the main, relay referred to, controls the operation of transmitting relays, which pass the signals to the Metropolitan or Provincial Sub-Chronopher. In a similar way local telegraph circuits are used for secondary distribution to the outlying offices, at which other apparatus is used to connect temporarily the subscribers' lines for the receipt of the signal. 115. The wires taken for Time distribution are selected from circuits which are normally used for telegraph purposes. The operation of the switching relays by the control clock has the effect of disconnecting the circuits from their ordinary telegraph apparatus, and transferring them to the transmitting relays by which the Time signals are sent. The arrangement is shown in skeleton form in Figure 44 and Figure 46. Figure 46 shows the use of a slightly more complicated arrangement than would be necessary in ordinary circumstances; additional relays having been introduced to allow certain Admiralty stations to send automatic return signals to Greenwich, so that the accuracy of the standard clocks may be verified. As it is imperative to limit the temporary suspension of traffic, use is made of "through" circuits in preference to a separate circuit to each station, so that several Sub-Chronopher stations may be served on one circuit. 116. In order to reduce the possibility of transmitting false signals, the main distribution circuits from London are transferred to the Chronopher half a minute before the circuits connected to the Sub-Chronophers in the Provinces are transferred. When the main distribution circuits pass through a repeater, it is necessary to insert at the repeater station the translating device shown in Figure 49, so that the repeater may be out out during the time the chronopher is in operation. At such repeater stations, if a Clock No. 24 or No. 12 is already installed, or an electric clock system exists admitting of the provision of a Clock No. 30, the switching is done by means of Relays, 90A controlled by such clock.
Greenwich Mean Time - Arrangements for repeating Time circuits to Admiralty stations, from which return signals are sent to Greenwich Observatory, are served directly from the Chronopher. The allocation of the main distribution circuits are arranged by the Engineer-in-Chief. Distinctive labels marked "Time Wire" should be fixed at all testing points on the main circuits connecting the Chronopher with the Sub Chronopher, where wires are liable to disturbance, and the necessity for the prevention of interruption to such circuits should be brought to the notice of all concerned. 117. The Sub-Chronopher It will be seen from Figure 50 that at secondary distribution stations (Sub-Chronophers) the switching relays are joined in three groups, for transmitting the 10.00 a.m., 1.00 p.m. or both signals, as may be required. It is not necessary for a galvanometer to be provided for each secondary line and no more than the three shown in Figure 50 should be fitted. Balancing resistances should be placed in the secondary lines in the positions shown. A 24-volt battery should be used for the switching relay and for the local relay circuits, but 80 volts, positive and negative, should be provided for the transmitting relay, unless for some reason a different voltage is more suitable. 118. Repetition of Time Signals Figure 50 illustrates the apparatus to be used for making these repetitions. When a failure is due to a fault on a main distribution wire another wire will be substituted at the Central Telegraph Office. Should the 10.00 a.m. signal fail from London, a signal at 1.00 p.m. will be sent by the Sub-Chronopher from the 10.00 am. relays. For this purpose the lead from the controlling clock to the 10.00 a.m. group should be temporarily joined (in parallel) with the connection to the 1.00 p.m. group, so that the 1.00 p.m. signal may be transmitted to all out-stations. This temporary connection should not be made until after the hour disc contact has opened, i.e., after 12.02 p.m. For repetition at 1.00 p.m. when only one secondary distribution circuit has failed at 10.00 a.m., a spare Relay 90A connected with the 1.00 p.m. group should be joined, to a spare set of line and instrument jacks on the test board. The connection between the inner spring of the jacks should be removed. Thus when the line and instrument jacks of the secondary distribution circuit concerned are joined to. the line and instrument jacks of the time repetition set, by means of pegs and cords, the circuit will work through the spare 1.00 p.m. switching relay, instead of through its normal 10.00 a.m. relay, until automatically transferred to the transmitting relay for the passage of time 1.00 p.m. signal. Repetitions at Sub-Chronopher stations of the 10.00 a.m. signal to local subscribers should be given similarly at 1.00 p.m. For this purpose the common connection of the 1.00 p.m. local subscribers should be joined to a spare jack at the test board, to which any subscribers' local circuit may be connected as required. If the 1.00 p.m. signal fails ,the repetition at 2.00 p.m. should be dealt with in a similar way. In this case it will be necessary to arrange for a repetition of the signal from the Central Telegraph Office. 119. Secondary Time Circuits 120. As a rule, hand-operated switches control the connection between public circuits and time circuits of subscribers who pay for the Time service. At the larger towns, where there are more than two subscribers, or at other places where there is any reason to doubt the ability of the staff to perform the manual switching operations satisfactorily, it may be desirable to provide for automatic switching between the public and private circuits. This should be borne in mind in connexion with the extension of the Time distribution system to new subscribers. With the growth of electric clock systems it will ultimately be possible materially to reduce the number of offices having manually operated switches. The standard apparatus and connections for manual switching at local offices is shown in Figure 51. The only offices at which changes have been made in one or other of these arrangements are those which serve Admiralty stations, and those which, in connexion with the electric clock system, or for other reasons as referred to in the preceding paragraph, have been, or can inure suitably be, equipped for automatic switching. 121. Switches, 3-bar, 3 position, Time, and Switches, 4-bar, 3 position, Time (Figure 51), are provided for the local end of the secondary Time distribution circuit. A 3-bar switch should be used on ordinary terminal circuits and it 4-bar switch on a through circuit, or on it circuit having intermediate apparatus at the office from which a renter is served. The 4-bar switch also provides a convenient means for connecting two circuits, each of. which is normally a terminal telegraph circuit, to form a through circuit with Greenwich Mean Time apparatus at the intermediate switching office. This course often saves disturbing another wire between the Sub-Chronopher station and a more distant office. 122. At offices where occasional, or periodical, extensions are made to other towns, for the convenience of Government Departments, a 4-bar switch should be substituted permanently for a 3-bar switch, and the earth connection shown on the "up" side of the relay obtained via the fourth bar of the switch (connected to a switch spring on the test-board), so that when the extension is required a peg and cord connection will convert the apparatus from a terminal to an intermediate set without disturbing the Time apparatus or modifying the ordinary switching operations. An instruction card, T.E. 224 should be placed in a conspicuous position near the Time apparatus at each local office. As the Time signals are sent automatically each day (including Sundays and holidays), it is necessary that the stall at local offices, terminal or intermediate, should be warned not to attempt to use, or to interfere with, the telegraph apparatus at the Time signal periods of ally day. Telegraph signals sent from a local office would interfere with the synchronizing arrangements at the Sub-Chronopher or automatic switching stations, and cause failures on the following day. The following instruction respecting the manipulation of the Time Switch is important. The switch should be turned to "Preliminary" at 9.58 a.m., or 12.58 p.m., as the case maybe. The preliminary signal, which is due between 9.58 and or 12.58 and 12.59 p.m., is indicated by a steady deflection of the needle of the galvanometer. The switch should not be turned to the "Time" position until this "Preliminary" signal has been received. The Time signal momentarily reverses the deflection; actuates the relay and consequently the galvanometer on the line. A steady current in the same direction as the preliminary current then occupies the distribution circuit for approximately 60 seconds. The switch is restored to "Inst" (shown as "Normal" on Figure 51) before the completion of the 60-seconds final current, applied by means of the automatic apparatus at the distribution centre. The risk of operating the Time Relay by telegraph signals, and consequent transmission of false signals, is thus prevented. 123. Relay, 90A
Relay, Type 90A (cover removed) It consists of an electromagnet B, having two 600 ohm windings which can be connected either in series or in parallel as required. This electromagnet has an armature A to which are attached two contact springs B and B1 connected to terminals T and T1 but insulated from each other and from the armature. These springs, when the relay is not energized, make contact with separate contact screws C which are attached to the terminals S. On the passage of it current the armature A is attracted downwards, carrying with it the contact springs B and B1. These make contact with the lower contacts D, which are connected to terminals M until the cessation of the energizing current, when they return to their normal position against the upper contacts C. Bias is given to the armature by the tension screw K and its associated spring. 124. Subscribers' Systems The Post Office Engineering requirements in 1898 advised of the following for ordinary requirements for sending Greenwich Time Signals to one subscriber in a provincial town. Apparatus required:- The current to Renter's Instrument must always be sent from 10 cells Leclanché (Porous Pot No. 3). The resistance of the Renter's Instrument should in all cases be approximately 200ohms.
Subscribers connection to GMT If the subscriber should propose to utilize the signals for the automatic synchronization, or correction, of a clock, the apparatus must be provided with means whereby the line circuit is normally connected to "earth," so as to short-circuit the receiving apparatus, except for the period of one minute, within which the Time signal is due to arrive. An added advantage will accrue if the switching device employed to achieve this purpose is also arranged to disconnect. simultaneously the whole of the electrical apparatus. This condition is imposed in order to prevent interference with the clock by false signals arriving at irregular times, such as might be caused by accidental contact between the subscriber's line and other wires, or by testing operations, etc. The Engineer-in-Chief's approval of any apparatus is not given unless, after examination, it is clear that the apparatus is not likely to give rise to complaints of failure in the Time signal due to inherent defects. It is important that this rule be rigidly observed, as there have been complaints of failure which have been subsequently traced to defects in apparatus which the subscriber alleged had been approved by the Post Office. Cases of doubt should be referred to the Engineer-in-Chief with a full description and, if possible, drawings of the proposed apparatus. 125. Hourly Signals for Clock Synchronization Distribution centres will be established in any town where there is a likelihood of reasonable support being obtained. All such proposals should be forwarded to the Engineer-in-chief by the Superintending Engineer, who will report on the prospective support, submitting a diagram and estimate for each local distribution circuit, together with mileage estimates and diagrams on Form T.E. 241, in respect of each subscriber's connection. Authority is required for local distribution circuits, but subscribers' spur circuits may be dealt with by Advice Note. It should be made quite clear to prospective subscribers to the synchronizing service that, although the master clock at the distribution centre is automatically synchronized to Greenwich Mean Time daily, the actual time received by the subscribers cannot be regarded absolutely as Greenwich Mean Time or correct to a fraction of a second. The subscribers' clocks would be corrected to a small fraction of a minute, which suffices for clocks indicating minutes. Where greater accuracy is desired, such as for clocks indicating seconds, chronometer rating, or for other scientific purposes, the Greenwich Mean Time Service is essential. Where a Clock No. 24 is used as a master clock for subscribers' clock synchronization, arrangements should be made for its synchronisation daily by the Greenwich Mean Time signal. The Synchronizing Service signals are derived from the hourly contact, providing for the commencement of the signal three minutes before the hour and its cessation exactly at the hour. The contact is arranged to operate double tongue Relays, 90A, and each such relay controls one local distribution loop (Figure 52). When the stock of Clock No. 24 is exhausted, Clock No. 36 in conjunction with Clock. No. 30 will be used. A condenser and resistance coil are joined in series across the master clock contacts for the purpose of preventing sparking. 126. Local Distribution Arrangements for Hourly
Synchronizing Service - Connections The test key is used for making periodic continuity tests round the loop. The function the function of the resistance is to admit of practical defections on the milliammeter and its value depends upon local conditions. Such tests should be made' at time when the circuit is not occupied by a synchronizing signal, i.e., when the subscribers spurs are disconnected. The line connection at the subscriber's end of the spur must be taken through contacts operated by a clock, so arranged that the apparatus used by the subscribe will normally be disconnected from the spur, except during it period from two, minutes before, until two minutes after, the time at which the signals are sent. I is necessary that the subscriber's clock should be regulated to maintain a rate inside this hourly limit. If the signals are to be used for synchronizing one clock only such as a regulator or a, master clock driving a system of electrical dials, the synchronizing apparatus, if suitable, may be connected directly in the line circuit The strength of the signals must not exceed 40mA . If the figure of merit does no allow of this a relay and local battery is required. When more than end clock is to be synchronized the subscriber should be required to install a local relay and a battery. In both instances the condition regarding the normal cutting of the line applies. In all cases the line coils of the subscriber's receiving apparatus must have a resistance of not less than 1,000 ohms. Normally, the maximum number of spurs to be connected to each distribution circuit is 10, which allows, with the standard apparatus, facilities for solving 50 subscribers from any one centre. If exceptional circumstances should arise to make it desirable to increase the number of subscribers connected to one distribution loop beyond the normal maximum of 10, the control clocks of the eleventh and subsequent subscriber must be arranged to join up their spur circuit at times other than the 58th minute of the hour. Additional subscribers up to a second group of 10 can then be served with the hourly signal at the 28th minute. Third and fourth groups of 10 may hay the signals at the 13th and 43rd minutes respectively. When the latter condition apply the number of subscribers in each of the groups should be balanced as is as possible. The maximum limit in these circumstances would thus be 40 subscribers connected to each distribution centre. The signals are then transmitted from the distribution centre every quarter of an hour, but successive signals are effective only to the subscribers in one particular group i.e., to a maximum of 10 of the 40 on each loop. In these circumstances it would be necessary for one, two, or three additional insulating striking pins to be added to the hour discs of the master clock. It is obvious that such additional pins should be planted with great accuracy. The disc should be adjusted on the clock so as to commence signals 3 minutes before the pre-determined times and to open the circuit precisely at the 60th minute for each group. This service of signals provides for synchronizing clocks in accordance with the "gaining rate" method, further particulars of which are given in connexion wit' the synchronization of mechanical clocks in Post Offices. The synchronizer described in connexion with that system are suitable for the use of subscriber but it is necessary for any subscriber's apparatus to be approved on behalf of the Engineer-in-Chief before the service is commenced. Where subscribers propose to use a typo of synchronizing apparatus operated on other than the "gaining rate", a full description of the apparatus, accompanied by a sketch or a detailed drawing should be submitted to the Engineer-in-Chief before tile apparatus is accepted. 127. Cessation of Time Signals to Places on the Coast Appendix D - Map of the distribution of GMT in the British Isles GMT Signal feeds - 1930 Other diagrams
|
||||||||
Last revised: January 30, 2025FM2 | ||||||||
By
means of mechanism, attached to the Mean Time Clock at Greenwich
Observatory, electrical signals are sent at the selected times through an
ground wire to the Central Telegraph Office (CTO) in London, where the signals
actuate a main relay which forms part of the Chronopher apparatus.
