Modern clocks depend on much more reliable frequencies to keep precise measurements. One consistent frequency that is almost available is that of the AC power grid, known as the utility frequency. In the United States, the utility frequency is 60 Hz, although many countries, particularly in Europe, use 50 Hz instead. Devices will not keep time, or possibly even operate, in an area with different utility frequency than that for which they were made, but they function reliably with minimal loss of precision on their local power grids. Any device powered by a wall socket can also use the oscillations of the current to keep time, and it is common for household appliances such as microwaves, ovens, and alarm clocks.

It is not typically feasible for a power provider to regulate frequency to exactly 60 Hz at all times, so clocks lose accuracy throughout the day. Fortunately, there is an elegant solution: each night, the frequency can be briefly increased or decreased to ensure that the correct number of periods occur in the day (5.184*10^6 on 60 Hz utility frequency) and no net time is lost or gained. However, this is still not accurate, or portable, enough for many applications, bringing us to the most common modern means of timekeeping: the crystal oscillator.

A crystal oscillator is an electronic component which generates a stable frequency using the mechanical resonance of any of a number of crystalline substances; quartz is the only one normally used in clocks. The first such clock was invented in 1927 by J.W. Horton and Warren Marrison. Size and cost both prevented them from becoming common until the 1980s, but they are now the most common timekeeping oscillator in production, found in most watches and clocks.

Quartz crystal resonator

The quartz oscillator from a common wristwatch. 32,768 Hz is the typical frequency for such an oscillator.

This method is many times more accurate than a mechanical clock; enough to provide a time signal in many electronic systems, but not enough to synchronize global communications and standardize world time.

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