This section is from the book "The Boy Mechanic Vol. 2 1000 Things for Boys to Do", by Popular Mechanics Co.. Also available from Amazon: The Boy Mechanic, Vol2: 1000 Things for Boys to Do.
It is frequently desirable in laboratory experiments, and in practica4 work as well, to maintain a constant level in a tank without allowing it to become full. In many cases an outlet pipe at a certain height in the side of the tank is not desirable, and in laboratory experiments with beakers or crocks is, of course, impossible.
The diagram shows a simple but effective constant-level device. The outer end of the inverted U-tube is curved upward so that it never empties. If desired, the upward curve may be omitted and the straight end immersed in a small vessel of water. All that is necessary now for the successful working of the device is that the inner or tank end, A, of the tube be lower than the outer end - in other words, below the level of the end B - and the inner end below the level of the fluid. Of course, the U-tube must be first filled with liquid and will then act as an intermittent, never-breaking siphon. Should the tank fill above the end B, the siphon drains the fluid down to that level and no lower, even if the inner leg of the tube reach the bottom. To maintain this level against loss by evaporation some slight inflow is necessary.
It will be noted that if the inner end of the siphon were above the outer end, the siphon would break as soon as the liquid in the tank fell to the inner mouth. - Contributed by Harry N. Holmes, Richmond, Ind.