How does a siphon work?

            A siphon is a device used to transfer a liquid from one container to another. It is shaped like a bent tube with arms of unequal length.

            A rubber or plastic tube is normally used as a siphon. Shaped like ‘U’ it has arms of unequal length. One end of the arm is placed in a container and the liquid is sucked up into the tube through the other end. A finger is quickly placed on the open end of the tube to keep the liquid in it. Keeping one end of the tube in the liquid, the open end is then placed in a second container, which should be necessarily at a lower level than the first container. You will find that soon the liquid starts flowing from the first container to the second.

            Now the question arises what makes the liquid flow from one container to the other?

            It is the air pressure which forces the liquid to flow from the first container to the second, provided the level of the liquid in the first container is higher than that in the second one. Because of the difference in the lengths of the arms, a pressure difference causes the liquid to flow. The liquid flows from one container to the other till the level of the liquid in the two containers becomes the same. Since the upper liquid is usually exposed to the atmosphere, the maximum elevation over which a water siphon will function is about 9 m (30 ft) which is the water equivalent of atmospheric pressure.