In commercial and industrial plants it is often necessary to remove a liquid in a small stream from a large cask, without disturbing a sediment, to fill smaller receptacles. This is particularly true in the case of corrosive liquids, like acids, ammonia, etc., where there is great danger in pouring the liquid from the cask. In such cases the task is accomplished by moans of a rubber tubing or bent glass tubing with unequal arms. This apparatus is called a siphon.

The principle of the siphon is explained as follows: In order to start the siphon it is necessary first to remove the air from it. This is done either by filling the siphon with water and placing a thumb at each end of the siphon, then placing the smaller end in the water that is to be removed from the vessel, or by drawing the water up through the long end of the tube. The water in the tube is driven toward the longer arm by a force equal to the difference in the weight of the water in the two arms. The difference in the lengths of the arms should be great enough to overcome the friction in the pipe and the weight of the water in the short arm. When this happens the water falls out of the long arm and tends to leave a vacuum at the top, but atmospheric pressure forces the water up the short leg to fill this space.

The tube ABCD (Fig. 60) is a siphon. The shorter leg AB is put into the liquid E, which is to be drawn off into G. If the air be taken out of the tube the pressure of the air on the surface of the liquid E will force the liquid up the tube AB, and it will then fill the whole tube and continue to run until all the liquid in E has run into the vessel G.