The design was first reported by McCarthy (1934). As shown in the diagram, a stoppered reservoir is supplied with an air inlet and a siphon. The pressure at the bottom of the air inlet is always the same as atmospheric pressure. If it were greater, air would not enter. If the entrance to the siphon is at the same depth, then it will always supply the water at atmospheric pressure and will deliver a flow under constant head height, regardless of the changing height of water within the reservoir.
This apparatus has many variations in design and has been used extensively when a constant water pressure is needed, e.g. supplying water at constant head for measuring water infiltration into soil or supplying the mobile phase in chromatography.