Candle clock is an ancient device for measuring the passage of time. The earliest reference is a Chinese poem by You Jiangu (AD 520). It appears in Wikipedia and other places, but I couldn’t find the poem itself, any help would be appreciated. These were simple clocks that were based on the relatively stable burning rate of candles. Linear graduation specified the elapsed time. All four candle clocks by al-Jazari are complex, full of inventions, a daring leap comparing to the classical candle clocks. In the clock of the Swordsman, the falcon emits a bronze ball every hour, so that the number of accumulated balls indicates the number of hours passed from sunset, at the same time the swordsman swings his sword and clips the top of the wick.
How does it work?
Al-Jazari opens the chapter of the candle clock of the swordsman with these words:
“I say that I have never come across a work by anyone on candle-clocks and have never seen a completed [example of such a] clock. I heard tell, however, of a candle-holder with a brass candlestick on it in which was a wax candle whose wick went through a hole in a cross-piece at the top of the brass candlestick. Near the foot of the candlestick was the head of a lion. When a constant hour had passed from the lighting of the candle, a ball fell from the mouth of the lion.”
The clock al-Jazari built was his version to the clock in the tale. The technical explanation, as always, will be colored in blue, so anyone who is not interested in balancing weights or bayonet mounts can skip those bits.
The massive candle, a height of about 40 centimeters (a span- شِبْر and a half) and almost four centimeters in diameter is standing on a cast bronze base. On the base, there is a brass sheath. The sheath is not a perfect circle but has two “lips” forming the ball channel, containing fourteen bronze balls. The candle alone blocks the balls. A heavy balancing weight of ~ 1.2 kg is connected through a pulley system. The weight guarantees that the bronze base and the candle are being pushed upward all the time. The length of the candle prevents the base from rising. The burning rate of the candle is measured meticulously, and the height of the candle is calculated so that it is suitable for sixteen hours of combustion, in practice, it would only burn for fourteen hours.
When the candle is lit at nightfall, the fire melts the wax, and after one hour the candle is shorter by 1/16. The weight will go down by this amount, and the base would go up, and the candle does not block any longer the lower ball. The ball is released and falls into the pouch attached to the string which is connected to the extension of the hand of the slave. As a result, the slave strikes the wick with his sword and cuts off the burnt-away section. The ball then rolls down and goes into the falcon’s head and then falls into the pedestal of the candle-holder. This happens at every hour until the end of the night
The part of the black slave and his sword is less detailed in my opinion, and its construction will require more experimentation and adjustments. Al-Jazari himself warns the reader that ” This movement [was perfected] after arranging and calculating and [after] repeated trials.”
Al-Jazari worked in the 12th century almost six centuries before Joseph Priestley discovered Oxygen, wax chemistry was also unknown at his time, and so was the understanding of Capillary Action. Despite this, his strong understanding of various materials, from working and experimenting, brings him to a few insights that we can now explain with the science we have learned. For example, al-Jazari requested that the candle will be made of pure wax. Candles can be prepared from natural fat, beeswax, whale fat, oil derivatives, and more. The rate of combustion depends on the combustion material, and as the material is more uniform, the rate of the combustion will be more uniform. He determines the weight of the wick, six grams. The wax is rising in the wick by capillary action, and therefore, various wicks will have different burning rate and would alter the time measurement.
One last thing, quite insignificant for the clock but interesting never the less. The candle cover was designed to replace candles comfortably. This method of mechanical attachment is known as “Bayonet mount,” and despite its exotic name, it is a useful technique of attachment to this day, for example in camera lenses or electric lights and includes a cylindrical male side with one or more radial pins, and a female receptor with matching L-shaped slots and with spring(s) to keep the two parts locked together:
The source of the peculiar name is the use of soldiers in this type of connection to quickly attach bayonets at the ends of their rifles, but the first documented bayonet mount is undoubtedly al-Jazari book in this chapter.
My chemistry teacher and Michael Faraday
In 1972, I was sixteen and studied in the Tichon Hadash in Tel-Aviv. It was the only year we studied chemistry. To my shame, I do not remember the name of my teacher, even though one of her classes is engraved in my memory as an extraordinary experience that affected me deeply. We were the second class to start our high school in the seventh grade before there were middle schools in Israel, we went through screening exams, and we were smart, at least in our own eyes and smugly knowledgeable. When the teacher said: “We would learn today about the candle.” The class broke into laughter; it seemed childish and not “scientific” enough for us. I’m afraid I’ve been among the laughing. Pretty soon she asked why the wax was burning up the wick and not burning in the candle? At once, in a fraction of a second, as in a revelation, I understood three things:
- First, that despite my laugher, I do not understand the candle burning at all.
- Secondly, there is a fascinating science in the most trivial things around us, like candles I have known well from Chanukah ceremonies and Shabbat candles.
- Third, I don’t ask questions, which If I were the young man I hope to be, I would ask.
That’s a lot for a single lesson. My chemistry teacher knew nothing about the internal storm I went through, and after years when I became a teacher myself, I thought about this lesson and I was hoping that sometimes I get to my students even when I don’t necessarily know about it.
When I worked at the Davidson Institute, Dr. Oved Kedem, my friend, introduced me to a thin book:
Michael Faraday was an English scientist, one of the best experimentalists in the history of science with an unusual life story. He was born into a poor family in London and was forced to help support the family as an apprentice in a local bookbinder and bookseller shop at age fourteen. He acquired all his education by reading books that were in the shop. The beginning of his scientific career was in popular lectures by Sir Humphry Davy, the president of the Royal Society at the time, so that his Christmas lectures were closing a circle. You can find the original book here. Those who do not want to deal with the original text can watch the series of short films done by Bill Hammack to present and explain Michael Faraday’s lectures.
On the fourth page, Michael Faraday answers the question of my chemistry teacher:
“Then there is another point about these candles which will answer a question—that is, as to the way in which this fluid gets out of the cup, up the wick, and into the place of combustion. You know that the flames on these burning wicks in candles made of beeswax, stearin, or spermaceti, do not run down to the wax or other matter, and melt it all away, but keep to their own right place. They are fenced off from the fluid below, and do not encroach on the cup at the sides. I cannot imagine a more beautiful example than the condition of adjustment under which a candle makes one part subserve to the other to the very end of its action. A combustible thing like that, burning away gradually, never being intruded upon by the flame, is a very beautiful sight, especially when you come to learn what a vigorous thing flame is—what power it has of destroying the wax itself when it gets hold of it, and of disturbing its proper form if it come only too near.”
This booklet is a real gem, and I met it much more experienced, after completing three degrees in science, but I was still fascinated and surprised by the opening sentence that opened the lectures, and I think that al-Jazari would be curious too:
“There is not a law under which any part of this universe is governed which does not come into play and is touched upon in these phenomena. There is no better, there is no more open door by which you can enter into the study of natural philosophy than by considering the physical phenomena of a candle.”