Before alarm clocks existed, people used nails fixed onto candles to wake up. they knew hoe to calculate the burning time and put the nail at right position. As the candle melted, nail fell, made a noise and woke its owner
There were much more sophisticated variants. In some monasteries they used metal balls stuck inside the candle, which rolled on the ground.
In other cases a rope was tied with a metal ring to the nail, and when the wax melted the ring would swing on the string and hit a metal plate several times.
(photo: de:Benutzer:Flyout)
German candle clock
A candle clock is a thin candle with consistently spaced marking that, when burned, indicates the passage of periods of time. While no longer used today, candle clocks provided an effective way to tell time indoors, at night, or on a cloudy day.
