Electric clock

An electric clock is a clock that is powered by electricity, as opposed to a mechanical clock which is powered manually by a hanging weight or a mainspring. The term is often applied to the electrically powered mechanical clocks that were used before quartz clocks replaced them in the 1980s. The first experimental electric clocks were constructed around 1800, but they were not widely manufactured until 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.

Contents 1 Types 2 History 3 Electromagnetic clock 4 Synchronous electric clock 4.1 Speed of motor 4.2 Accuracy 4.3 Spin-start clocks 5 Notes 6 References

Types

Electric clocks can operate by several different types of mechanism:

• Electromechanical clocks These have a traditional mechanical movement, which keeps time with an oscillating pendulum or balance wheel powered through a gear train by a mainspring, but use electricity to rewind the mainspring with an electric motor or electromagnet. This mechanism is found mostly in antique clocks.
  • Electric remontoire clocks In these, the gear train was turned by a small spring or weighted lever, called a remontoire, which was wound up more frequently by an electric motor or electromagnet. This mechanism was more accurate than a mainspring, because the frequent winding averaged out variations in the clock's rate caused by the varying force of the spring as it unwound. It was used in precision pendulum clocks, and in automotive clocks until the 1970s.
• Electromagnetic clocks keep time with a pendulum or balance wheel, but the pulses to keep it going are not provided by a mechanical movement and escapement linkage, but by magnetic force from an electromagnet (solenoid). This was the mechanism used in the first electric clocks, and is found in antique electric pendulum clocks. It is also found in a few modern decorative mantel and desk clocks.
• Synchronous clocks rely on the 50 or 60 Hz utility frequency of the AC electric power grid as a timing source, by driving the clock gears with a synchronous motor. This type of clock was widely used from the 1930s to the present.
• Quartz clocks are electric clocks which keep time with a vibrating quartz crystal. They are the most common type of clock today.

History

In 1814, Sir Francis Ronalds (1788) of London invented the forerunner of an electric clock, the electrostatic clock. His prototype was powered with a dry pile battery. It proved unreliable in timekeeping, however, because of a strong dependence on a stable room temperature and 'weather conditions'.

In 1815, Giuseppe Zamboni (1776-1846) of Verona invented and showed another electrostatic clock run with dry pile battery and an oscillating orb. Over the test of time Zamboni's clock was praised "the most elegant and at the same time the most simple movement yet produced by the electric column".^[1] 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 is the first to invent and patent the electric clock. 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 electromechanically 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.^[2][3] The first commercial synchronous electric clock sold in the UK, the Synclock, was brought out in 1931.^[3]

Electromagnetic clock

The configuration of this device is comparatively very simple and reliable. The typical deviation of the electromechanical clock is about 10 seconds per hour and it usually can run for about a year using the energy of the single battery. The electrical 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 opens the current across the second inductor, and the second inductor works as 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 is absent or does not move. The resonance frequency of the mechanical oscillator should be about several times per second.

Synchronous electric clock

A synchronous electric clock doesn't contain a timekeeping oscillator like a pendulum, but instead relies on the oscillations of the AC utility current from its wall plug to keep time. It consists of s small AC synchronous motor, which turns the clock's hands through a reduction gear train.^[4] The motor contains electromagnets which create a rotating magnetic field which turns an iron rotor. The shaft of the motor makes one revolution for each oscillation of the current, so the rotation rate 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.^[4]

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.

Because they don't contain a delicate pendulum or balance wheel, synchronous clocks are very rugged. However, they are vulnerable to power outages. A power interruption will cause the clock's hands to stop moving, so they will be behind the correct time when the power resumes. If a temporary power outage occurs while the owner is out, the running clock will not show the correct time when he returns. For this reason, many synchronous clocks have a "power interruption indicator", on the face, a colored disk operated by an electromagnet, that turns red if the power has been interrupted, to indicate that the clock needs to be reset. Turning the setting knob resets the indicator.

Speed of motor

The above describes a synchronous clock with a two-pole motor, whose motor runs at 3600 RPM (60 Hz models) or 3000 RPM (50 Hz models).^[5] Some electric clocks have motors in which the rotor has more magnetic poles (teeth). Therefore the motor shaft rotates slower, at a smaller multiple of the line frequency. The advantage of this design is that fewer gears are required to reduce the speed of the motor shaft to turn the hands. The speed of a synchronous motor v in revolutions per minute (RPM) is related to the number of poles by:

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 (60 Hz models) or 200 RPM (50 Hz models).

Accuracy

Electric utilities keep the long-term frequency of their current very constant, calibrated by UTC atomic clock time, so synchronous clocks will keep accurate time, although short-term fluctuations in the frequency may cause varying errors of a few seconds during the course of a day. For example, European utilities fine-tune the frequency of their grid once a day so that the total number of cycles of alternating current is exactly 50 Hz × 60 sec × 60 min × 24 hours = 4,320,000 per day, so clocks will not accumulate significant error. 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.^[4] 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.

Notes

References

• Viradez, Michel. History of Electric Clocks
• Katz, Eugenii. Alexander Bain Biography
• Perpetual Electromotive of Giuseppe Zamboni
• Chirkin, K. Electromechanical clocks. Radio, 7 (1968): p. 43.