Ancient Greek technology

I had the chance to visit the Museum of Ancient Greek Technology, an unassuming facade on an Athens street. The works were beautifully presented. The cleverness and simplicity of the pieces showcase just how advanced the ancient Greeks were during their time, with many principles still in use today. A selection of interesting pieces are below.

1. Pythagorus cup

This cup has a special design which punishes the greedy – when you fill it up past a certain point, the liquid begins to empty out of the bottom, covering you in wine. Contrary to what you might expect, the flow doesn’t stop at the ‘acceptable’ level but instead will continue until the cup is empty.

As you fill the cup, the pressure at the surface of the liquid is equal to the air pressure, ie zero relative to the air. This is true both inside and outside of the centre section, since both surfaces are in contact with the air. This is why their heights are always the same during pouring.

The pressure at the bottom of the cup, relative to the air, is equal to the height of the liquid x the density of the liquid x the gravitational acceleration = hpg. You can think of it as a function of h, = f(h), since the other two values are constant. At this point the pressure at the bottom out the outer and inner sections are equal ie f(h_outer) = f(h_inner) because the heights are the same. So there is no liquid flow.

If the user keeps filling the liquid, eventually the liquid starts to flow over the inner divider, because h_inner has become fixed while h_outer has kept increasing so the unequal pressure distribution causes a flow of liquid. In other words, the liquid ‘wants’ to get to the same height as the liquid in the outer section, but is limited by the cup’s geometry.

In the last image, the liquid keeps flowing because the pressure at the bottom of the cup stem zero (relative to the air pressure) yet at the bottom of the cup is still hpg. There is an uneven pressure distribution, and therefore flow of liquid, until the cup drains completely.

This principle is exactly how modern siphons in toilets work when you flush. That’s why they have that odd curved pipe shape at the back.

2. The first crane

When the Greeks discovered mechanical advantage, they used it for hauling huge rocks and material for construction, basically inventing the crane. Mechanical advantage is the idea that you can lift a heavier load with the same force if you stand further away from the pivot point between you an the load. For example, a seesaw can be balanced with two people of different weights just by placing the lighter person further from the centre. In a crane, the ‘light person’ is the human, and the ‘heavy person’ is the load, who can be moved into the air when the ‘light person’ (human) adjusts their position further from the pivot.

3. The hydraulic clock – Clepsydra

This invention is an incredibly clever piece of engineeering. The clock is able to operate continuously on its own all year, with the water provided from a local stream and flowing into the top container. The middle container has a level controlling system consisting of a conical valve, which supplies a dripper. This water drips into the bottom container, whose float (the hemisphere) rises with the water level. The float is connected to a pointer which indicates the time of day on the top chart. At the end of 24 hours, the water drains rapidly via the siphon on the right – exactly as it does in the Pythagorus cup above.

The clock is also capable of showing the day of the year. This is via a system of gears that rotates the top chart by 1/365 turns every time the bottom container is drained, ie once a day.