Electric clock

Telechron synchronous electric clock manufactured around 1940. By 1940 the synchronous clock became the most common type of clock in the U.S.

An electric clock is a clock that is powered by electricity, as opposed to a mechanical clock which is powered by a hanging weight or a mainspring. The term is often applied to the electrically powered mechanical clocks that were used before quartz clocks were introduced in the 1980s. The first experimental electric clocks were constructed around 1840, but they were not widely manufactured until mains electric power became available in the 1890s. In the 1930s the synchronous electric clock replaced mechanical clocks as the most widely used type of clock.

Types

Electromechanical self-winding clock movement from Switzerland.

Electric clocks can operate by several different types of mechanism:

History

One of Alexander Bain's early electromagnetic clocks, from the 1840s

In 1814, Sir Francis Ronalds (1788-1873) of London invented the first electric clock.[3] It was powered with dry piles, a high voltage battery with extremely long life but the disadvantage of its electrical properties varying with the weather.[4] He trialled various means of regulating the electricity and his later models proved to be reliable across a range of meteorological conditions.[5]

In 1815, Giuseppe Zamboni (1776-1846) of Verona invented and showed another electrostatic clock run with dry pile batteries and an oscillating orb. His team produced improved clocks over many years, which were later denoted as "the most elegant and at the same time the most simple movement yet produced by the electric column".[6] Zamboni's clock had a vertical needle supported by a pivot and was so energy efficient that it could operate on one battery for over 50 years.

In 1840, Alexander Bain (1811-1877), a Scottish clock and instrument maker was the first to invent and patent a clock powered by electric current. His original electric clock patent is dated October 10, 1840. On January 11, 1841, Alexander Bain along with John Barwise, a chronometer maker, took out another important patent describing a clock in which an electromagnetic pendulum and an electric current is employed to keep the clock going instead of springs or weights. Later patents expanded on his original ideas.

Numerous people were intent on inventing the electric clock with electromechanical and electromagnetic designs around the year 1840, such as Wheatstone, Steinheil, Hipp, Breguet, and Garnier, both in Europe and America.

Matthias Hipp (1813-1893), clockmaker born in Germany, is credited with establishing the production series, mass marketable electric clock. Hipp opened a workshop in Reutlingen, Switzerland, where he developed an electric clock to have the Hipp-Toggle, presented in Berlin at an exhibition in 1843. The Hipp-Toggle is a device attached to a pendulum or balance wheel that electro-mechanically allows occasional impulse or drive to the pendulum or wheel as its amplitude of swing drops below a certain level, and is so efficient that it was subsequently used in electric clocks for over a hundred years. Hipp also invented a small motor and built the chronoscope and the registering chronograph for time measurement.

The first electric clocks had prominent pendulums because this was a familiar shape and design. Smaller clocks and watches with a spiral-balance are made on the same principles as pendulum clocks.

Henry Ellis Warren (1872-1957) invented the first synchronous electric clock which kept time from the oscillations of the power grid, in 1918.[7][8] The first commercial synchronous electric clock sold in the UK, the Synclock, was brought out in 1931.[8]

Electromechanical Clock

photograph of Master Clock
Master clock from synchronized school clock system. c.1928 Electromechanical movement winds each minute and impulses slave clocks each minute. Operates on 24 Volts DC

A clock that employs electricity in some form to power a conventional clock mechanism is an electromechanical clock. Any spring or weight driven clock that uses electricity (either AC or DC) to rewind the spring or raise the weight of a mechanical clock then is an electromechanical clock. In electromechanical clocks the electricity serves no time keeping function. The timekeeping function is regulated by the pendulum. Near the end of the nineteenth century, the availability of the dry cell battery made it practical to use electric power in clocks. The use of electricity then led to many variations of clock and motor designs. Electromechanical clocks were made as individual timepieces but most commonly were used as integral parts of synchronized time installations. Experience in telegraphy led to connecting remote clocks (slave clocks) via wires to a controlling (master clock) clock. The goal was to create a clock system where each clock displayed exactly the same time. The master and the slaves are electromechanical clocks. The master clock has a conventional self-winding clock mechanism that is rewound electrically. The slave clock mechanism is not a conventional clock mechanism as it consists only of a ratchet wheel and time train. Slave clocks rely upon electrical impulses from the master clock to mechanically move the clock hands one unit of time. Synchronized time systems are made up of one master clock and any number of slave clocks. The slave clocks are connected by wires to the master clock. These systems are found in locations where multiple clocks would be used such as learning institutions, businesses, factories, transportation networks, banks, offices and government facilities. A notable example of this type of system is the Shortt-Synchronome clock, which is an example of an electromechanical gravity remontoire. These self-winding clock systems were usually low voltage DC. They were installed thru the 1950s and by then systems with synchronous motor clocks were becoming the clock system of choice.

Electromagnetic clock

Early French electromagnetic clock

The configuration of this device is comparatively very simple and reliable. The electric current powers either a pendulum or an electromechanical oscillator.

The electromechanical oscillator component has an attached magnet that passes two inductors. When the magnet passes the first inductor or sensor, the simple amplifier causes the current through the second inductor, and the second inductor works as an electromagnet, providing an energy pulse to the moving oscillator. This oscillator is responsible for the accuracy of the clock. The electronic part would not generate electrical pulses if the oscillator was absent or did not move. The resonant frequency of the mechanical oscillator should be several times per second.

Synchronous electric clock

Clock radio with synchronous clock, from the 1950s

A synchronous electric clock does not contain a timekeeping oscillator such as a pendulum or balance wheel, but instead counts the oscillations of the AC utility current from its wall plug to keep time. It consists of a small AC synchronous motor, which turns the clock's hands through a reduction gear train.[9] The motor contains electromagnets which create a rotating magnetic field which turns an iron rotor. The rotation rate of the motor shaft is synchronized to the utility frequency; 60 cycles per second (Hz) in North and South America, 50 cycles per second in most other countries. The gear train scales this rotation so the minute hand rotates once per hour. Thus the synchronous clock can be regarded as not so much a timekeeper as a mechanical counter, whose hands display a running count of the number of cycles of alternating current.[9]

One of the gears turning the clock's hands has a shaft with a sliding friction fitting, so the clock's hands can be turned manually by a knob on the back, to set the clock.

Synchronous motor clocks are rugged because they do not have a delicate pendulum or balance wheel. However, a temporary power outage will stop the clock, which will show the wrong time when power is restored. Some synchronous clocks have an indicator which shows if it has stopped and restarted.

Number of poles

Some electric clocks have a simple two-pole synchronous motor which runs at one revolution per cycle of power, i.e., 3600 RPM at 60 Hz and 3000 RPM at 50 Hz.[10] However most electric clocks have rotors with more magnetic poles (teeth), consequently rotating at a smaller submultiple of line frequency. This allows the gear train which turns the hands to be built with fewer gears, saving money.[11]

Accuracy

Electric utilities keep the long-term average frequency of their current very constant, calibrated by UTC atomic clock time, so synchronous clocks keep accurate time in the long term and do not accumulate error, although short-term fluctuations in the frequency caused by utility load variations may cause errors of a few seconds during the course of a day. For example, European utilities control the frequency of their grid once a day to make the total number of cycles in 24 hours correct. U.S. utilities correct their frequency once the cumulative error has reached 3-10 sec.

Spin-start clocks

The earliest synchronous clocks from the 1930s were not self-starting, and had to be started by spinning a starter knob on the back.[9] An interesting flaw in these spin-start clocks was that the motor could be started in either direction, so if the starter knob was spun in the wrong direction the clock would run backwards, the hands turning counterclockwise. Later manual-start clocks had ratchets or other linkages which prevented backwards starting. The invention of the shaded-pole motor allowed self-starting clocks to be made, but since the clock would restart after a power interruption, the loss of time would not be indicated.

Notes

  1. ↑ Elliott Sound Products: build a synchronous clock
  2. ↑ Brimarc, typical quartz clock movement specified accurate to within ±30 seconds/month
  3. ↑ Aked, C.K. (1973). "The First Electric Clock". Antiquarian Horology.
  4. ↑ Ronalds, B.F. (2016). Sir Francis Ronalds: Father of the Electric Telegraph. London: Imperial College Press. ISBN 978-1-78326-917-4.
  5. ↑ Ronalds, B.F. (Jun 2015). "Remembering the First Battery-Operated Clock". Antiquarian Horology. Retrieved 8 Apr 2016.
  6. ↑ Perpetual Electromotive
  7. ↑ U.S. patent #1283434 Warren, Henry E. Timing device, filed February 26, 1917, issued October 29, 1918, on Google Patents
  8. 1 2 "Famous Names in Electrical Horology". Electrical Horology Group. Antiquarian Horological Society, London, UK. 2011. Retrieved 2011-12-16.
  9. 1 2 3 Wise, S. J. (1952). Electric Clocks, 2nd Ed. (PDF). London: Heywood & Co. pp. 95–100.
  10. ↑ Wise (1952) Electric Clocks, p.101-104
  11. ↑ The speed of a synchronous motor v in revolutions per minute (RPM) is related to the number of poles by:
    v = \frac {120f}{p}\,
    where f is the line frequency (50/60 Hz) and p is the number of poles on the rotor. Many designs have 30 poles, so that the motor runs at 240 RPM (at 60 Hz) or 200 RPM (at 50 Hz).

References

See also

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