One of my co-workers went to a conference in Greece,
and came back with a present,
of something which is very old,
from 400 years B.C. or even earlier,
but I'd never seen before.
It's called the Pythagoras Cup
and it has this strange bit sticking up in the middle.
And the story is
that this cup was invented by Pythagoras.
The one that worked on right-angled triangles.
He invented it so when he gave his students wine,
that they all got the same amount
and if somebody was greedy,
that student
got his comeuppance
It behaves normally,
until you get to this gold ring here, the one I am running my finger around,
not the one one up here.
And if you fill it above that,
that's somewhere between these two,
suddenly,
all the liquid in the glass
drains out
and through a hole in the bottom.
Of course, you can't see what's happening here, so I rushed off
and bought a glass one
so you can see what's happening.
I asked Neil to demonstrate it
You pour in the water,
imagine it's wine, but if it was wine you couldn't see inside.
And, as the level of wine rises,
suddenly, if you were too greedy,
it all dribbles out.
"sound of water draining"
And, it siphons out through the little tube in the centre.
The way it works
is that inside here there is a u-tube
which has a small opening at the bottom.
And as the liquid rises,
the u-tube fills up
and when it gets to the top,
it siphons 'round
and sucks the water out.
I was really excited about this,
and then when I was driving home,
I wondered, what would happen if you filled it with mercury.
Mercury is a very dense liquid.
So I persuaded Neil to fill it with Mercury,
and it works just as well with Mercury.
And,
what is interesting
is that when you do it with water,
the water on our glass
Pythagoras Cup
dribbles out at a sort of angle
because water so-called 'wets' glass; it sticks to the glass.
But with mercury, because it is denser,
and it doesn't wet glass, it goes straight down.
And makes quite a sort of spectacular
pattern in the bottom of the dish.
I then had a slightly strange idea:
I wondered what would have happened if you put just a little mercury in
and then filled the rest with water.
And I predicted, because water is much less dense than mercury,
that, in fact, you could fill the cup right to the top.
So Neil obligingly tried it even though he was meant to be going on holiday
and it works.
You can fill the Pythagorus Cup right to the top if you put a bit of mercury at the bottom
So what it happening is that the weight of the water
is just not enough to force the mercury up inside the siphon.
If you fill it completely with mercury, you have enough weight pushing down
to make the siphon work.
So the upshot is
that if Pythagoras' students had been clever,
they could've put some mercury in the bottom of the cup
and filled it right to the top.
The downside of course,
is that they would have poisoned themselves.
There were quite nice crystals that you could see formed
in the surface of the metal.
And there was this great lump of mercury at the bottom, solid.
But Neil had quite a lot of difficulty getting it out at the end
and it began to melt and so on.
But that wasn't part of the experiment.