An inverted U-tube with one limb longer than the other. Liquid will be transferred from a reservoir at the base of the shorter limb to the end of the longer limb, provided that the U-tube is filled with liquid. The device is useful for emptying an inaccessible container, such as a car’s petrol tank.

The pressure (p₁) on the liquid at the base of the short limb (length h₁) is p−h₁k, where p is the atmospheric pressure and k is a constant equal to the product of the density of the liquid and the acceleration of free fall. The pressure (p₂) on the liquid at the base of the long limb (length h₂) is p−h₂k. Thus for fluid flow to occur through the tube, from short limb to long limb, p₁ > p₂, and for this to occur h₂ > h₁. Thus if the limbs are of equal length no flow will occur; it will only occur if the limb dipping into the reservoir is shorter than the delivering limb.

Siphon.