What is a siphon?
A siphon is a continuous tube — usually bent into an upside-down U — that moves liquid from a higher container to a lower one. The surprising part is that the liquid has to travel uphill first: it rises up the short arm, crosses the highest point of the bend, and then falls down the long arm into the lower container.
On this diagram the upper basin on the left holds orange liquid at a higher level, while the cyan liquid in the lower basin on the right sits lower. A single tube links the two.
Reading the diagram
The blue dashed lines inside the tube trace the direction the liquid flows. The light blue arrows pressing down on both liquid surfaces are atmospheric pressure. The red dashed line between the two surfaces marks the height difference that keeps the siphon running.
Why it matters
Once it is primed, a siphon needs no pump or power — gravity and air pressure do the work. That is why it is used to drain fish tanks, empty flooded cellars, move fuel between containers and even to gauge flow in water clocks and flow meters, all with nothing more than a filled hose.
1. Two basins at different heights
The upper basin shows a tall column of orange liquid; the lower basin shows a shallow layer of cyan liquid. Because the upper surface is much higher than the lower one, there is a height difference between them — the energy source that will drive the siphon.
2. The tube is primed
For a siphon to work, the tube has to be completely filled with liquid, with no air pockets. Filling it like this is called priming. The full, sealed tube links the two liquid surfaces so that pressure can be passed from one to the other.
3. Atmospheric pressure presses on both surfaces
Air pushes down on the surface of the liquid in each basin, shown by the light blue arrows. Because the tube is full, this push is carried through the liquid to the whole length of the tube, not just the surface.
4. The taller column weighs more
The column of liquid on the lower-basin side is taller than the one on the upper-basin side. Being taller, it weighs more, so it pulls down harder. The two pushes at the surfaces are balanced, but the weights are not — the heavier column wins.
5. Liquid climbs the bend and flows
With the heavier column pulling, liquid is drawn up the short arm, over the bend and down the long arm, exactly as the blue dashed lines show. As long as the tube stays full, the flow is continuous.
6. Flow stops when the levels meet
The siphon keeps draining the upper basin into the lower one. As it runs, the upper level falls and the lower level rises, so the height difference shrinks; when the two surfaces are level there is nothing left to drive the flow, and the siphon stops by itself.
What is a siphon?
A siphon is a continuous tube with a bend in it that moves liquid from a higher container to a lower one. Once the tube is completely filled, the liquid flows up the short arm, over the bend and down the long arm into the lower container — even though part of the journey is uphill.
Why does the liquid flow uphill over the bend?
Atmospheric pressure pushes down on both liquid surfaces. Because the tube is full and sealed by the liquid, that push is carried through the liquid. The taller column on the lower side weighs more than the column on the higher side, so the balance tips and liquid is drawn over the bend towards the lower basin.
Why does a siphon need to be primed first?
The liquid can only be pushed over the bend if the tube is already full. If any air is trapped inside, the air breaks the continuous link between the two liquid surfaces and the push from atmospheric pressure cannot be passed along. Filling the tube — priming it — removes the air and lets the flow start.
Does atmospheric pressure have to be strong?
It only has to be strong enough to lift the liquid over the bend. Atmospheric pressure can support a water column of about 10 metres, so for everyday siphons over a metre or two the air pressure is more than enough.
Why does the flow stop on its own?
The siphon keeps running while the upper liquid level is higher than the lower one. As liquid drains, the two levels move towards each other; when they become equal there is no height difference left to drive the flow, and it stops.
Does the shape or length of the tube matter?
The tube only needs to be continuous and stay full of liquid. A fatter or thinner tube changes how fast the liquid moves, but it is the difference in height between the two liquid surfaces — not the shape of the tube — that decides whether the siphon runs at all.