The highest point of a siphon is called-

Explanation:

• A syphon is a long-bent pipe used to connect two reservoirs at different levels through a high ridge. The highest point of the syphon is called the summit. Or
• A pipe which rises above its hydraulic grade line has negative pressure and is known as syphon.

• As the point C is above the free surface of the water in the tank, the pressure at C will be less than the atmosphere. Theoretical the pressure at C may be reduced to -10.3m of water but in actual practice, it is -7.6m of water or 10.3-7.6 = 2.7m of water absolute.
• If the pressure at C becomes less than 2.7m of water absolute then air and other gases from water would come and accumulate at the summit and obstruct the flow.

Important Points

• Summit pressure should not be less than vapour pressure of the liquid to avoid air lock formation at the summit (For water it is 2.5m at 20°c)