> A variation on this is used to check the authenticity of some
At least in NA, mains frequency is very very stable. The entire
continent is in phase and clocked with very high precision. It's more
stable than a free-running digital clock.
...I used this fact to 'cheat' in a high school physics contest where
we had to build a clock and the winner was the most stable. Ours had
a CdS cell that watched the 120Hz flicker of the lights in the room.
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