Slave clock

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In telecommunication and horology, a slave clock is a clock that depends for its accuracy on another clock, a master clock. Many modern clocks are synchronized, either through the Internet or by radio time signals, to a worldwide time standard called Coordinated Universal Time (UTC) based on a network of master atomic clocks in many countries. For scientific purposes, precision clocks can be synchronized to within a few nanoseconds by dedicated satellite channels. Slave clock synchronization is usually achieved by phase-locking the slave clock signal to a signal received from the master clock. To adjust for the transit time of the signal from the master clock to the slave clock, the phase of the slave clock may be adjusted with respect to the signal from the master clock so that both clocks are in phase. Thus, the time markers of both clocks, at the output of the clocks, occur simultaneously.[1]

Before the computer era, the term referred to satellite electrical clocks that are synchronized periodically by an electrical pulse issued by a master clock. From the late 19th to the mid 20th centuries, electrical master/slave clock systems were widely used in public buildings and business offices, with all the clocks in the building synchronized through electric wires to a central master clock.

These older styles of slave clocks either keep time by themselves, and are periodically corrected by the master clock, or require impulses from the master clock to advance. Many slave clocks of these types remain in operation, most commonly in schools.

Diagram of electric time system used around 1910 to keep time in factories, schools, and other large institutions. The master clock (bottom center), controlled by a temperature-compensated mercury pendulum, is wired to slave clocks throughout the building. In addition to wall clocks, it also controls time stamps that are used to stamp documents with the time, and a turret clock used in a clock tower. The "program clock" is a timer that can be programmed with punched paper tape to ring bells or turn machines on and off at preprogrammed times.

Synchronous Systems[edit]

This type of system was developed by the International Time Recording Company (ITR) in the early 1900s. This company was later renamed International Business Machines (IBM), which sold their time division to the Simplex Time Recorder Company in 1958. The methods & clock mechanisms have remained basically unchanged, other than appearance and style over the years. Today, this same protocol is used by several manufacturers, including Lathem, Dukane, Cincinnati and Standard Electric Time. Digital master clocks have replaced older mechanical types.

All consist of a master clock and a number of secondary clocks in remote locations. These secondary clocks appear to run like an ordinary plug-in electric clock and run on their own synchronous electric motor and will maintain correct time unless there is an interruption in electric service, depending on the master clock only for the periodic synchronizing impulse. These clocks differ from the ‘impulse’ type, which are electrically advanced each minute by the master clock. Impulse type clocks are usually called ‘slave’ clocks since they cannot function at all without the master clock.

There are two basic variations in synchronous systems, ’wired’ and ‘electronic’.

In a wired system, the secondary clocks are ‘hard-wired’ to the master clock with their own network of wiring. This dedicated wiring consists of 3 wires and requires that all secondary clocks be connected to this network in order to receive corrections from the master clock. These systems are usually 110 volts but some are 24 volts. The master clock energizes the third wire to apply voltage to the correction solenoids (called ‘clutch magnets’ by the manufacturer) in the secondary clocks.

In an electronic system, the correction impulse is applied to the normal building wiring, in the form of a high-frequency audio signal (typically 3510 Hz) applied to the normal 60 Hz frequency of the electrical system. Special receivers in the secondary clocks receive this impulse & apply voltage to the correction solenoid. In this type system, secondary clocks need no special wiring & may be plugged into any 110 volt outlet in the building.

The cycle can also be initiated manually for testing & adjustment as follows:
With clock running, rotate 90 degrees to the right, so that the ‘9’ is at the top. Hold in this position approximately 8 seconds (you may hear a clicking sound during this time), then return the clock to its normal upright position. Within 60 seconds, the hands will slowly advance to the next hour as described below in the third paragraph under ‘Hourly Correction’.

The actual correction that follows is the same in both types of systems. The only difference (described above) is the method by which the correction solenoid is energized.

Hourly Correction[edit]

The correction impulse is issued by the system master clock at 57 minutes 54 seconds after each hour and lasts for 8 seconds (until 58 minutes 02 seconds). The correction solenoids in the secondary clocks are energized by this pulse. This causes the attached armature and cam to engage a gear and rotate upwards to lift a latch allowing the correction cycle to begin. This takes 6 seconds and causes the secondary clocks to begin their correction cycle at precisely 58 minutes 0 seconds.

The correction cycle lasts precisely 60 seconds, ending at 59 minutes 0 seconds at which time, all clocks in the system will display the same time as the master clock.

The cycle causes an internal clock-wise sweep, beginning at the 59 minute position and takes 60 seconds, rotating around the dial, ending at the 59 minute position. During this sweep, if the minute hand is not in proper position, when the sweep rotation reaches it, the minute hand is slowly rotated at 1 rpm until it reaches the 59 minute mark. At this point it is now in sync with the master clock.

During the cycle, the red second hand continues its normal 1 rpm motion but is held in place when it reaches the 60 second mark. It is released when the cycle is complete (minute hand reaches 59 minutes) and now minutes and seconds are both in sync with the master clock.

12 Hour Correction[edit]

This impulse is issued by the system master clock at 5:57:54 AM and PM and is a longer version of the hourly correction impulse. Instead of lasting 8 seconds, this impulse is of 14 seconds duration, lasting until 58 minutes, 08 seconds. The cams in the secondary clocks are rotated further and lock the correction cycle latch open, causing the cycle to continue until the clocks reach 5:59:00. Depending on how many hours the secondary clocks were slow, they will now be several minutes slower than the master. This error is then corrected by the next normal hourly correction at 6:57:54.[2]

Impulse Slave Clocks[edit]

An older variation of the master clock system used once-a-minute electric impulses to advance slave clocks. Early system ran on batteries in the master clock. These batteries wound the master clock and provided the necessary voltage to advance the slave clocks once a minute. The first manufacturer of this system was The Standard Electric Time Company, originally of Waterbury, Connecticut, but later Springfield, Massachusetts.

At the 59th second of every minute, the master clock closed a series circuit, energizing the magnet in each of the slave clocks. At the top of the minute, the circuit was opened and the magnets released. This advanced the minute hand on each of the slave clocks by one minute. The voltage used depended on the size of the clock system, the bigger the system, the higher the voltage. Slave clocks were wired in a series circuit.

Later systems (beginning in the late 1920s) were wired in parallel and typically ran on 24VDC, though smaller systems may have run on a lower voltage. International Time Recorder (later IBM) was the first to market with a self correcting system, an impulse was energized on a third wire to advance the slave clocks to the top of the hour. During the 59th minute, extra impulses were sent (approximately one per second) on the primary wire pair, which caused any clock that was slow to catch up at the 59th minute. If the clock was fast, it would stop at the 59th minute and just sit there until the impulse came over the third wire, causing the clock to go to the top of the hour. This approach eventually became the "industry standard" for impulse slave clocks in the 50s right up through today.

The Standard Electric Time Company used a different approach. At the 59th minute, a 10-second impulse was sent between the 14th and 24th second of the minute, which released the slave clock mechanism which in turn, with the help of a weight attached to the minute hand, caused the minute hand to swing up to the 59th minute and lock into place when released. This would correct and clock that was 25 minutes fast or slow. If the clock was more than 25 minutes fast or slow, the minute hand would swing up to the closest hour, which could make the clock either an hour fast or slow, depending on the location of the minute hand at the time of correction. This correction scheme was modified later to use only two wires, with 48VDC used for correction and 24VDC used for the normal minute impulse. Unlike the IBM/Simplex correction system, Standard Electric Time clocks with this type of correction can be used with no correction impulses at all, they'll just impulse through the top of the hour with a 24VDC impulse, contrary to what most modern master clock manufacturers claim in their literature, as many of today's manufacturer claim a correction impulse is required to take the clock to the top of the hour when this simply is not the case.


Mechanical slave clocks from the 1950s and 1960s era.


  1. ^  This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C".
  2. ^ National Association of Watch and Clock Collectors posting

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