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Impulse clock systems

Gents master clock

Before the advent of quartz electronic clocks, which enabled low-cost clocks to offer excellent timekeeping performance and run for years on a battery, there were two types of electric clock offering high accuracy that were suitable for use in quantity throughout large premises. Synchronous clocks, powered by the AC mains electrical supply, use the precisely-controlled mains frequency to keep time. In contrast, impulse clock systems used one accurate master clock to control any number of remote slave dials over a dedicated low-voltage circuit.

Synchronous clocks offered the advantage of inherently reliable timekeeping. The electrical supply company took over responsibility for this function by maintaining their average network frequency within very close limits. Offset against this were a couple of nuisance factors: Early synchronous clocks were not generally self-starting, so even a minor glitch in the power would require a visit to every clock in the building to set it going again. Even with self starting movements, each clock needed individual re-setting after a power cut or when changing to and from daylight-saving time. In a building with hundreds of clocks, some of which might be hard to reach, this could be an onerous task! In areas with DC mains power, synchronous clocks were simply not an option.

Impulse clock systems were designed to operate large numbers of clocks, spread throughout a site, in an efficient and convenient way, by remotely controlling the time shown on every dial using electrical impulses. A very accurate electrically-driven master clock (usually a longcase pendulum timepiece) would be installed in a convenient place accessible to the maintenance staff. Most systems arranged for the master to send an impulse every thirty seconds over one or more circuits, each of which could operate up to around forty slaves. The slave movements could be ordinary wall-mounted dials, timeswitches, time recorders or even tower clocks; in each case the slave movement would be a simple, reliable, low cost mechanism.

Timekeeping could be monitored and regulated easily at the master clock, and so long as the system was functioning normally one could be fairly certain that the time displayed on every dial was identical. The annual changes to and from daylight saving time needed to be carried out only at the master clock; the slave dials would follow. Impulse clock installations use very little electrical power, so sufficient battery-backup was normally included in the specification to keep the system running for days in the absence of mains. Disadvantages included the unavoidable stoppage of all clocks if the master clock or wiring became faulty, and the distinctive 'click' made by most types of slave dial every thirty seconds. For greater precision, where a sweep second hand was required on the slaves, a one-second impulse system was available; this lacked much of the convenience of the 30-second system as each slave needed individual setting.

The master clock

The Pul-Syn-Etic master clock by Gents is typical of British 30-second master clocks of its era. Time is kept by a 2-second invar pendulum which advances a special 15-tooth escapement wheel having one notch deeper than the others. Every 30 seconds the ratchet click drops into the deep notch, in which position it allows a trigger arm to release a catch at the next swing, causing the 'gravity arm' to fall. Attached to the gravity arm is a roller that strikes an inclined pallet on the pendulum, keeping it in motion by giving it an impulse that depends only on the weight and travel of the gravity arm. This configuration ensures the the impulse does not vary with battery voltage and hence has a predictable effect on the timekeeping; furthermore the impulse occurs when the pendulum is near the centre of its swing as in this position it causes the least variation to the period. When the gravity arm has fallen it closes a contact completing the circuit to impulse the slave clocks via the master clock magnet. This attracts an armature arranged to restore the gravity arm onto its catch, then overtravel and break the circuit. The inertia of the gravity arm causes the contacts to remain closed for the correct impulse duration, sufficient to operate slave movements of all types.

Master clock and relay panel

Master clock and relay panel

Half-minute wheel

Half-minute wheel

Master clock movement

Master clock movement

Within the clock case is a resistance for regulating the circuit current and a dial mounted in the door from which the circuit time can be seen. This is an ordinary electric slave dial that has no mechanical connection to the pendulum movement. This particular slave dial includes a special repeater contact for supplying impulses to time recorders, many models of which required one impulse per minute rather than every 30 seconds. A 60-tooth wheel is arranged to close a contact on alternate 30-second periods, in series with a contact operated by the armature. This generates a 1-minute pulse of the same duration as the incoming 30-second pulse from the master.

Slave dials

Slave movement detail

Slave movement detail

Ordinary slave movements consist of a magnet coil with a pivoted armature arranged to advance a ratchet wheel. This usually has 120 teeth enabling it to drive the minute hand directly, making for a very simple and durable movement. Slave coils were normally of low resistance, all being connected in series with the master clock and battery. This configuration offered the advantages of fast response (on account of the low inductance), robustness of the windings and the certainty that all coils would receive the correct current regardless of the wiring resistance. Care is required, however, to ensure that the circuit is not broken when a slave is removed for maintenance etc. The voltage burden imposed on the circuit by each slave depends on the size of its coil, and this on the size of the dial, but all were made to operate on a standard current (typically 250-350 mA depending on manufacturer) enabling diverse types of movement to be connected in series in a single circuit up to the maximum voltage, typically 60 volts, that the master clock could safely impulse. Where more slaves were required than one circuit could supply, the system would be divided into sections controlled by sub-transmitting relays from the impulses in the master clock circuit. So that faults in the relay circuits could be identified easily, it was conventional to include tell-tale dials for all circuits, grouped in a distribution panel. These would normally show the same time as the master, but in the event of one circuit being interrupted (by a wiring fault for example) the corresponding tell-tale dial would stop.

Dual circuit panel

Dual circuit panel

Triple circuit distribution panel

Triple circuit distribution panel

These two Synchronome sub-transmitting panels were located in a switchroom at the London Guildhall; the master clocks were long gone by the time these were rescued. The larger, older unit contains a relay operated by the master clock circuit in series with its slave dial circuit termed the 'home loop'. This relay provides impulses to all five circuits fed from both panels. Each circuit includes a slave dial, a variable resistor to adjust the current and a switch to select normal or 'advance' mode, allowing any or all circuits to be impulsed manually using the advance pushbutton. For convenience during current adjustment, a jack is provided to which an ammeter can be connected, a suitable meter with flylead being provided at the top of each unit. There is an interesting subtlety in the arrangement of the sub-transmitting relay coil within the home loop. Because the coil is in series with the home loop slave dials, any break in that circuit would affect all the sub-circuits as well, by preventing the relay operating. To overcome this vulnerability a further relay is included in series with the slaves, which when operated disconnects a dummy load resistance equal in value to the total slave resistance. Thus in normal operation both relays energise and the slaves are fed with their proper current. If the home loop slaves fail to draw current then the substitute resistance stays in circuit to pass the necessary current through the sub-transmitting relay coil.

Adjustable programmer

Pul-Syn-Etic programmer

Pul-Syn-Etic programmer

One useful device often fitted to impulse clock systems was a programmer, in effect a versatile timeswitch. Programmers were often used for ringing bells to signal shift changes in factories and lesson changes in schools, and judging by the graffiti on the case our Gents Pul-Syn-Etic model C69 seems to have been used for school bells. Basic programmers with factory-set ringing times were available for applications where the times would not normally need to be changed, but this unit allows for adjustment in 5-minute steps throughout the day, along with automatic omission of weekends. Ringing times are set by inserting taper pins into the holes in the large cast 24-hour dial. When the slave coil impulses every thirty seconds, it advances an hour cam with twelve 5-minute teeth. A special mercury switch is tilted up stepwise as the pawl rises up the sawtooth, the mercury being guided away from the contacts as it flows through a capillary tube in the switch vial. When, at the beginning of the next 5 minute period, the pawl drops free from the sawtooth, the mercury surges along the vial, bridging the contacts and completing the circuit. The mercury then flows slowly through the capillary into another chamber and after eight seconds the circuit is broken. In fact three separate contacts must be closed for the bell to ring: The omit dial must be standing at a weekday to close its contact, a pin must be inserted in the 24-hour dial for the current 5-minute period to close the time setting contact, and eventually it is the mercury switch that makes and breaks the bell current. This series (logical AND) arrangement of contacts of differing cycle length provides a convenient way to set a programme with a period of one week to a precision of thirty seconds, without demanding unreasonably tight mechanical tolerances. Variants of this unit were available with up to four channels, although at reduced resolution.

Programmer movement

Programmer movement

5-minute contact

5-minute contact

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