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TantalusCup_05-15_RN

While Ctesibius may not be as famous as Archimedes, Aristotle, and Pythagoras, he certainly deserves recognition as one of the world’s first and best engineers. A prolific inventor living in the third century BC, Ctesibius is often referred to as “the father of pneumatics.” He created the first truly accurate water clock, a compressed-air catapult, a water pump, a musical organ that ran on water, and a number of remarkable machines that made use of siphons.

At one time historians credited Ctesibius with inventing the siphon, but we now know it’s much older. Around 1400 BC, during the reign of Pharaoh Amenhotep II, Egyptian engravers etched pictures on the walls of tombs at Thebes. One of these shows a group of aficionados siphoning wine from several containers into a large punch bowl, presumably to produce a blend of superior flavor and bouquet. The ancient Egyptians probably also used the siphon for purifying water, irrigating crops, and drinking beer.

But it was the classical Greeks, and Ctesibius in particular, who really explored the scientific principles by which siphons work. We know few details about Ctesibius’ life. We do know that he was the son of a barber, and he rose from humble beginnings to become the head of the Library of Alexandria, which at the time was the greatest on Earth.

Ctesibius seems to have had a particular fondness for hydraulics and pneumatics, as well as a wry sense of humor. He put the siphon to work in a number of amusing inventions, including mechanical singing birds, a religious statue that could alternately stand and sit while being carried through the streets, and the “engibita,” apparently automatons that could seemingly drink water.

Siphons are sort of mysterious devices. Most people, if they have an opinion on them at all, believe that siphons work because of a difference in air pressure between the upper and lower levels of the water reservoirs. But that’s not the whole story, because a siphon can work even in a vacuum, with certain exotic liquids. (There’s a terrific video at makezine.com/go/siphon that shows this.)

So, if air pressure isn’t the only reason, then how does a siphon work? There are two other factors: the force of gravity and the attraction between molecules in a liquid, which is called cohesion. To understand this, think of a siphon as an inverted U with one leg longer than the other. The weight of the fluid in the longer leg is obviously greater than the weight of the fluid in the shorter leg. As the fluid in the longer leg falls out due to the pull of gravity, the molecules in the shorter leg are acted upon by gravity and cohesion. In a siphon, air pressure and molecular attraction win out over gravity and push/pull the fluid up and over the hump.

The Tantalus Cup

The Tantalus Cup combines the workings of a siphon with a droll sense of humor. Doubtlessly Ctesibius would have found it amusing. It’s sometimes called a Greedy Cup or Pythagoras Cup despite the fact that Pythagoras certainly did not invent it.

The cup works normally when partially filled. But if the user fills the cup beyond a particular level, a hidden siphon empties the cup. Far cleverer than a run-of-the mill dribble glass, it’s a combination science lesson and practical joke in one easy-to-make package. Ancient versions had a small figure of Tantalus inside; alas, wine would never reach his lips.

Diagrams-ABC

You can make the siphon 3 ways. Option A is easier, but Options B and C have a nicer appearance.

Related

Steps

Step #1: Prepare the cup.

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Ctesibius and the Tantalus CupCtesibius and the Tantalus CupCtesibius and the Tantalus Cup
  • Drill a 1/4" hole lengthwise through the stem of the drinking cup.
  • Next, file several openings in the bottom lip of the cup, to provide an exit for the liquid when it drains out.

Step #2: Make the siphon (Option A).

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Ctesibius and the Tantalus CupCtesibius and the Tantalus CupCtesibius and the Tantalus CupCtesibius and the Tantalus Cup
  • Option A: Pill bottle.
  • As shown in Diagram A, insert the 2-1/2" tube about 1/2" into the hole you drilled. Fix it in place with the silicone sealant, then seal the joint all around.
  • File a 1/4"×1/4" opening in the lip of the bottle. Apply sealant to the lip (don’t plug up the opening you just cut), invert the bottle over the tube, and press it into the bottom of the cup. Let dry.

Step #3: Make the siphon (Option B).

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Ctesibius and the Tantalus CupCtesibius and the Tantalus CupCtesibius and the Tantalus Cup
  • Option B: Plastic tube.
  • Following Diagram B, insert the 1"-long aluminum tube halfway into the plastic tube, then insert this assembly into the hole in the cup. Seal the joint with silicone and let dry.
  • Bend the plastic tube into a loop, taking care not to kink it, until its open end is ¼" above the bottom of the cup. Fix with a cable tie.

Step #4: Make the siphon (Option C).

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Ctesibius and the Tantalus CupCtesibius and the Tantalus CupCtesibius and the Tantalus CupCtesibius and the Tantalus Cup
  • Option C: Glass tube.
  • Buy a 6" length of glass tubing if possible. If not, use the edge of a steel file to notch the tubing, wear heavy gloves to break it to length, then sand or fire-polish the sharp broken ends.
  • Mount and light a propane or butane torch securely, donning heatproof gloves. Referring to Diagram C, rotate the area of the tube to be bent in the center of the flame until it softens. Remove and quickly bend it around the metal rod to form a 180° U.
  • Once the tube is cool, insert it into the hole in the cup and seal the joint with silicone. A polycarbonate wine glass adds some class.

Step #5: Using the Tantalus Cup.

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Ctesibius and the Tantalus Cup
  • The legend of the Tantalus Cup is that it was designed to teach moderation. The cup appears normal enough, except for the column or figure inside.
  • If you pour a moderate amount of wine into the cup, you can drink out of it in the normal fashion. But if you fill it to the brim (actually, to any level over the height of the siphon assembly) the cup will empty itself. Every last drop will siphon out, and you won’t get a drink at all.

Conclusion

This project first appeared in MAKE Volume 35, page 134.
William Gurstelle

William Gurstelle

William Gurstelle is a contributing editor of MAKE. The new and improved edition of his book Backyard Ballistics is out now.


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