What it shows:

A siphon is a device that allows the transfer of a fluid from one reservoir to a second at a lower level even though the first part of the journey is up-hill.

How it works:

A siphon is effectively an inverted U-tube with unequal length tubes. The asymmetry means that there is a pressure difference between the ends;

at the upper reservoir: p1 = P - ρgh1
at the lower reservoir: p2 = P - ρgh2
(where P = atmospheric pressure)

so p1 > p2 if h2 > h1 and the fluid flows. To get the siphon 'running' you do need to reduce p2 and the best way is orally by sucking on the tube (this is why we use water rather than gasoline which would have been be easier to see), but once the head difference is achieved the process is automatic.

figure 1: principle of the siphon


Setting it up:

The upper reservoir is a 2L beaker which sits on the bench. The lower is a bucket, and the siphon itself a 2m rubber hose with a 1cm bore.


Siphons only work if the height h1 is less than the barometric height (not a problem here), although exceptions can be made if the liquid is pure and cohesive molecular forces come into play.