Communicating Vessels

• To understand the physical principle of the communicating vessels
• Stevino's law

Observing the level reached by the liquid in the four glass tubes of the apparatus, one can check the so-called communicating vessels principle. According to it, the level of the liquid surface in a set of tubes communicating with one another is the same in all tubes if the liquid contained in each tube is the same.

In fact, since at equilibrium pressure must be the same at all points of the liquid at the same depth, the height of the liquid columns in the various tubes with respect to some horizontal plane must be the same. If the tubes contain the same liquid, the heights of the surfaces are the same for the tubes containing the same liquid.

It can be immediately verified that the level of the water surface is the same in all the tubes and it remains so even if the apparatus is inclined (if the water level is such that in the second tube the surface is in the curved section, the separating surface with air is slightly inclined; this depends on the presence of a surface tension determining the patched angle between the glass surface and the water).

By partially emptying the apparatus from water and by substituting water with a non-miscible liquid (for instance oil) it can be seen that, while the levels in the tubes containing water coincide, the level in the tube containing the other liquid is different.

Since the specific weight is defined as g we have that with this apparatus it is possible to obtain the measurement of the specific weight of a non-miscible liquid with respect to water’s specific weight.

• Communicating vessels — 4180.60
• Food colouring — 4207.56
• Base for capillary tube and communicating vessel — 5405.30

• Does the level of the liquid filling the system depend on the shape?
• If we added a non-miscible liquid to water, what would we observe?