Original clocks have a set of points in them, similar to a turn signal flasher. The points opening/closing is what causes the clock hands to "jump", or step, instead of having a smooth sweep. These points are usually what causes the clock to stop working, as they burn over time, just like the points in your ignition system, for those of us with point type ignitions.
The points burning/wearing, and sticking closed is also a big factor in draining the battery overnite, and in several cases, has been the cause of fires in the car.
There are quartz movements that do kinda-sorta mimic the "step" of an analog clock, but the steps are much quicker.
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