Starting around the 6th century BCE, Greek thinkers made the first great strides toward understanding the physical world. First came the astronomers, led by Thales of Miletus, who were followed by the mathematicians, such as Pythagoras and Euclid. Once the basic methods of scientific thinking were in place, the tap of knowledge was turned wide open, producing a stream of great thinkers in quick succession, including the famous scientists Archimedes, Aristotle, and a scientist who you may not know (yet), Ctesibius of Alexandria.
The Jokester of Alexandria
Although Ctesibius may not be as well known as Greek scientists such as Archimedes, Aristotle, and Pythagoras, this Greek certainly deserves recognition as one of the world’s first great engineers. He was, apparently, also a fellow who liked a good joke. He invented a moving statue that was carried on a cart in festival parades. The crowds lining the street were amazed to see the statue stand up and sit down, like a real person!
Ctesibius lived in the 3rd century BCE and is often referred to by modern scholars as “the father of pneumatics.” Pneumatics is the engineering discipline that uses gases to make things move. One area in which Ctesibius’s contributions were especially important was in machines that use siphons.
At one time, historians actually credited Ctesibius with inventing the siphon, but we know from more recent archeological studies that the siphon is much older. In 1500 BCE (during the reign of Pharaoh Amen-Hotep, and more than 1,000 years after Ptah-Hotep), Egyptian engravers etched several notable pictures on the walls of tombs at Thebes. These pictures show some of the earliest evidence that humans knew how siphons work.
One of these drawings shows a group of wine aficionados siphoning wine from several containers into a large punchbowl, presumably to produce a blend of superior flavor and bouquet. Beyond wine preparation, it appears that the ancient Egyptians used the siphon for many household purposes, including to purify drinking water.
Archeologists have also found that a few hundred years later, the Assyrians, under King Sennacherib, built a massive aqueduct to bring water to the then burgeoning city of Nineveh. A hill stood between the city and its water source, so a huge siphon was constructed to lift the water over it. This astounding piece of early engineering was conceived so masterfully that even modern hydraulic engineers could not improve on the technique until the 19th century.
Although siphons have been used for a very long time, it was the classical age Greeks, and Ctesibius in particular, who most carefully explored the scientific principles through which they work.
Even though siphons have been used since the time of ancient Egypt (and possibly well before), it seems that modern scientists still can’t agree about the exact forces that make siphons work.
Many experts insist that siphons siphon because of the atmosphere. They contend that when a liquid within the siphon tube is “sucked” through the tube and over a higher point than at either end, the suction is due to a difference in atmospheric pressure between the high and low points in the tube. But when other scientists tested a siphon in a vacuum, they found it still worked. So some other explanation must be necessary.
Perhaps, say other scientists, they key ingredient is gravity, not air pressure. When liquid is sucked up the tube and over the hump, it’s the force of gravity that continues to pull the liquid through the tube. Others say that small but numerous bonds between molecules pull other liquid molecules up and over. This theory, called the chain model, points to a phenomenon called liquid cohesion as the reason siphons work.
It may be that elements of all of these theories, or perhaps another not yet considered, may be at play. What we do know for sure is that siphons do work, and it’s not easy to explain why.
But despite Ctesibius’s explorations and the generations of scientists who followed him, the exact mechanism through which siphons work, as the diagram above shows, is still something of a mystery.
Like the siphon itself, we don’t have all the details about Ctesibius’s life. We do know he was probably either a barber at one time, or the son of a barber, and that his cleverness and ingenuity were such that he became quite famous. He rose from humble beginnings to become the head of the Library of Alexandria, which, at the time, was the greatest library on earth.
Ctesibius seemed to have a particular fondness for hydraulic and pneumatic things and siphons in particular. And, because he was a clever man with a wry sense of humor, he put the siphon to work in a number of different amusing inventions, including mechanical singing birds, a water-powered musical organ, and the engibita, which was apparently an automaton that could drink water and perform other lifelike acts.
The next project you’ll build, called the Tantalus Cup, combines the science of siphons with a droll sense of humor. Did Ctesibius actually invent it? Well, no one can say for sure, but doubtlessly this is something that he would have found very amusing. And, his early research on how siphons work is the basis for how this cup works.
Designing and Building a Tantalus Cup
The Tantalus Cup is sometimes called a Pythagoras Cup despite the fact that Pythagoras did not invent it. The device itself is quite a clever joke: the cup works and acts like any other cup when it isn’t filled with too much wine (or other liquid). But if the user fills the cup beyond a particular level, a hidden siphon empties the cup. So, if you get too greedy with your wine or juice, the cup teaches you a lesson in restraint and humility! Far cleverer than a run-of-the-mill dribble glass, it’s a combination of a science lesson and a practical joke in one easy-to-make package.
Part 1: Preparing the Cup
To start the process of making your Tantalus Cup, follow these steps:
1. Begin by drilling a ¼” hole through the stem of the drinking cup as shown in Figure A.
2. Next, file several openings in the bottom lip of the glass to make places for the water to exit (see Figure B).
Now you’re ready to make the siphon.
Part 2: Making the Siphon
Before you begin, take a look at Figure C to get an idea of how this will work. As you’ll see, you can make the siphon in three ways. Option A is slightly easier, but options B and C have nicer appearances.
Option (A): The Plastic Bottle Method
1. As shown in the first cup in Figure C, insert the 2½” tube into the hole you drilled into the stem of the drinking cup in Step 1.
2. The tube should extend about ½” into the hole. Fix it into place with the silicone sealant. Take care to spread the sealant so there is no leak where the tube enters the hole in the cup.
3. While the silicone dries, use the file to make an opening in the lip of the pill bottle that is approximately ¼” by ¼” (see Figure D).
4. Next, apply sealant to the lip of the pill bottle (taking care not to plug up the opening you just made with the file) and invert the pill bottle.
5. Press the pill bottle firmly onto the bottom of the cup and let it dry.
Option (B): The Plastic Tube Method
The second cup in Figure C shows how to assemble this project.
1. Insert the 1-inch-long aluminum tube halfway into the plastic tube and then insert the aluminum and plastic assembly into the hole that you drilled in the bottom of the drinking cup in Step 1.
2. Add silicone sealant to the joint.
3. Let it dry.
4. Bend the plastic into a loop so the open end is ¼-inch above the bottom of the cup. Fix it into place with a cable tie (see Figure E).
Option (C): The Glass Tube Method
1. Light a propane or butane torch.
Note: Propane torches must be used according to the manufacturer’s instructions. They should be used only by adults or under close adult supervision.
2. Don heat-proof gloves.
3. Refer to the last cup in Figure C to see what you’re aiming for and then rotate the section of the tubing to be bent as you hold it in the center of the flame.
4. When the glass becomes soft, remove the tube from the flame and quickly bend it around the metal rod so it forms a 180-degree turn (see Figure F).
5. Once the tube is cool, insert the long leg of the glass tube in the hole as shown in Diagram C. Add silicone adhesive to the joint and let it dry.
Exploring Siphons with the Tantalus Cup
The legend of the Tantalus Cup is that it was designed to teach moderation. The cup appears normal, except for the copper coil or plastic column inside. If you pour a small to moderate amount of water or wine into the cup, you can then drink out of it in a normal fashion. But if you fill the cup to the brim (actually, to any level over the height of the siphon assembly) the cup will empty itself! Every last drop will drain out from a hole in the stem, and you won’t get a drink at all.
The trick is that there is a hidden siphon in the cup. When the water level is below the level required for the siphon to actuate, nothing happens—the cup is just a cup. But if the cup’s owner adds so much liquid that the level rises above the activation point, the siphon kicks in and the glass empties out completely, dribbling its contents out the bottom of the cup via secret holes.
Let that be a lesson to live in moderation!