When synchronous clocks were first introduced there was extensive discussion as to whether it was best to have a self starting clock or a non self starting clock.
The self starter had the advantage that it would restart after a power failure (not uncommon in those days) but the disadvantage that the owner did not know if it had stopped, in which case the time shown would not be correct.
On the other hand the non self start clock stopped if there was a power cut so the owner knew it was not right.
With the latter type a telltale of some sort was often fitted so the owner could see if it was still running. This was often a wheel with painted sectors, or a seconds hand. With the former type, the self start models, a different telltale was sometimes fitted which would show an indication, perhaps a patch of red showing in a dial aperture if the clock had been stopped at any time.
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