The Pythagorean Cup and Siphons

A rather thought-provoking invention, Pythagorean’s cup is like any other cup except for a small mechanism located in its center.

This mechanism consists of a hollow pipe-like chamber that follows an opening, starting from the bottom of the liquid-holding part of the cup, up to the top of the central column that makes up the cup’s core, and back down 180 degrees, out the bottom.

As the pipe curls over the top of the central column, after coming up from the entrance of the cup, its floor marks an imaginary line of sorts. If this line is extended horizontally in both directions, it will trace out a “limit” across the liquid-Illustration of pythagorean's cupholding section. This limit is the point at which, if filled further, liquid will begin to siphon out the bottom and onto your lap or feet…Yowza!

See, if the cup is only filled till below this limit or level, you’ll never have a problem. It’s when you get greedy and begin pouring too generously, that you end up with a wet lap and an empty cup. Due to hydrostatic pressure, once the liquid closes the air passage at the top of the column and begins falling downward, it will “drag” or siphon a trail of said liquid until the cup’s contents are completely emptied.

This phenomenon called the siphon is created due to a reduction in pressure at the higher end of the siphon. As gravity pulls the water column down the pipe of the Pythagorean cup, the lower pressure thus created on the other end causes the liquid there to be overpowered, subsequently allowing itself to be “dragged” along, stopping only when the water level either falls below the intake, or the outlet.

If the level falls below the intake, the introduction of air will break the siphon; and if the level falls below the outlet, the pressures at the two siphon Illustration of Siphonends will reach an equilibrium – which consequently grinds the siphon to a halt. Unless one of these two eventualities occur, the “ideal” and undisturbed siphon can theoretically continue indefinitely without assistance.

And you should know that a normal siphon doesn’t require liquid tensile strength in order to work. In other words, even if there is a break or an air pocket within the siphon, where there is absolutely no connection between the water, the siphon can still operate normally. The important things are that the high end is submerged in the liquid and that there is a pressure difference. However, once the siphon has begun, no additional energy is required to keep the flow going – it can now run entirely on autopilot.

Priming the Siphon

You may have experience “priming” a siphon by mouth, and while this works and is suitable for many situations, it may not always be feasible. Another common way to accomplish this is by first plugging up the lower end of the siphon and filling it with liquid. You then submerge the higher end into the upper reservoir. By simply releasing the plug from the lower end, you can kick-start the siphon by utilizing the hydrostatic pressure.

This method is particularly useful when siphoning toxic substances such as gasoline using a simple hose. You should know that attempting to orally prime a siphon submerged in gasoline or any other toxic liquid is potentially dangerous and discouraged. The possibility of either swallowing or aspirating the substance (into the lungs) is high, both of which can cause serious injury to your throat, stomach, and lungs, resulting even in death.


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