Abstract: An ultraviolet purification system in which ultraviolet radiation is generated by an ultraviolet lamp wherein the amount of ultraviolet radiation is varied in accordance with fluid flow. The intensity of the ultraviolet lamp is controlled by a circuit which is responsive to fluid flow for selectivity energizing the lamp to provide variable UV intensity output depending upon fluid flow. The circuit arrangement is such that the ultraviolet lamp is always started with a high current ballast. The selection of ultraviolet lamp intensity outputs according to fluid flow maximizes the useful life of the ultraviolet lamp and renders the system more efficient by reducing the amount of heat transmitted to the fluid being acted upon during periods of low flow or non-use.