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.
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. They essentially count cycles of the power supply. While the actual frequency may vary with loading on the grid, the total number of cycles per 24 hours is maintained rigorously constant, so that these clocks can keep time accurately for long periods, barring power cuts; over months they are more accurate than a typical quartz clock. This was the most common type of clock from the 1930s but has now been mostly replaced by quartz clocks.
Tuning Fork clocks keep time by counting the oscillations of a calibrated tuning fork with a specific frequency. These were only made in battery-powered form. Battery-powered clocks have been made using the schemes above with the obvious exception of a synchronous movement. All battery-powered clocks have been largely replaced by the lower cost quartz movement.
Quartz clocks are electric clocks which keep time by counting oscillations of a vibrating quartz crystal. They use modern low-voltage DC-powered circuitry, which may be supplied by a battery or derived from mains electricity. They are the most common type of clock today. Quartz clocks and watches as supplied by the manufacturer typically keep time with an error of a few seconds per week, although sometimes more. Inexpensive quartz movements are often specified to keep time within 30 seconds per month (1 second per day, 6 minutes per year). Lower error can be achieved by individual calibration if adjustment is possible, subject to the stability of the oscillator, particularly with change in temperature. Higher accuracy is possible at higher cost.
Radio-controlled clocks are quartz clocks which are periodically synchronized with the UTC atomic clock time scale via radio time signals broadcast by dedicated stations around the world. They are distinct from clock radios.