You Jiangu's device consisted of six candles made from 72 pennyweights of wax, each being 12 inches high, of uniform thickness, and divided into 12 sections each of one inch. Each candle burned away completely in four hours, making each marking 20 minutes. The candles were placed for protection inside cases made of a wooden frame with transparent horn panels in the sides. Similar methods of measuring time were used in medieval churches and earlier, famously by King Alfred the Great of England, first by counting the number of candles of a specific size burnt, and later by use of a graduated candle.
The most sophisticated candle clocks known to date, however, were those of Al-Jazari in 1206. It included a dial to display the time and, for the first time, employed a bayonet fitting, a fastening mechanism still used in modern times. Donald Routledge Hill described one of al-Jazari's candle clocks as follows:
The candle, whose rate of burning was known, bore against the underside of the cap, and its wick passed through the hole. Wax collected in the indentation and could be removed periodically so that it did not interfere with steady burning. The bottom of the candle rested in a shallow dish that had a ring on its side connected through pulleys to a counterweight. As the candle burned away, the weight pushed it upward at a constant speed. The automata were operated from the dish at the bottom of the candle.
- Turner, Anthony J. The Time Museum, Volume I, Time Measuring Instruments; Part 3, Water-clocks, Sand-glasses, Fire-clocks