AC Electric Clocks
After the development of the modern power grid, clocks could be synchronized through power lines. Before quartz movements, this was perhaps the most accurate type of common clock.
AC Power
The dawn of the power grid as we know it now began in the 1880s. For about 10 years there were fights over what sort of current should be used. That fight was called the War of the Currents. DC systems were first to be installed, but AC systems allowed easy power transmission over great distances. AC systems also allowed the voltage to be stepped down for safer use in homes.
By the mid 1890s the war was mostly over, and AC systems became the common system used throughout the world.
Most AC power systems in North America use a frequency of 60 cycles per second. Most of the rest of the world uses a frequency of 50 cycles per second.
The higher the frequency the less iron is needed in transformers and motors, so all other things being equal, 60 cycle systems are less expensive to build. Aircraft use 400 cycles per second in order to reduce weight even more.
Note that the invention of the connection between a pendulum and the gear train of a clock allowed the beat of the pendulum to regulate the accuracy of the clock. Modern AC grid power systems can be harnessed to the same end.
Even more important, many AC power grid operators have traditionally ensured that the total count of cycles in a calendar day are correct. These cycle counts are traditionally set against national time standards bodies.
So using the frequency of the AC power grid as a source for time keeping has been a common way to build inexpensive yet very accurate clocks. These clocks never need winding and they only need to be set when there is a power failure.
This type of clock does, of course, require a reliable electric grid. Different electric grids, even in advanced countries, have very different reliability numbers. Areas subject to severe weather tend to have less reliable AC power grids.
Synchronous Motors
Motors build to run directly on AC power come in various types. One type of AC motor is called a synchronous motor. This name means that the rotation of the motor's shaft is synchronized to the individual waves of electricity coming into the motor from its power source.
So by connecting the shaft of a synchronous motor to the hands on a clock face, the clock itself becomes synchronized to the AC power grid and thus back to national standards bodies.
Before the widespread adoption of battery powered quartz movements, it was very common for table and wall clocks to be built using this type of motor.
Use in Bible Clocks
Simple Bible Clocks are easier to build using quartz movements. Being battery powered there is no concern for electrical safety using these movements. But, for some advanced forms of Bible Clocks, synchronous AC motors may be a good choice. Especially if the clock involves heavy use of specialized gears. These might include clocks with second hands accurate to scripture. This might also include clocks with calendar indicators in their set of dials.