A
ballcock (also
balltap or
float valve) is a
mechanism or machine for filling water tanks, such as those found in
flush toilets, while avoiding overflow and (in the event of low water pressure) backflow. The modern ballcock was invented by
José Antonio de Alzate y Ramírez, a Mexican priest and scientist, who described the device in 1790 in the Gaceta de Literatura Méxicana. It consists of a
valve connected to a hollow sealed
float by means of a
lever, mounted near the top of the tank. The float is often ball-shaped, hence the name
ballcock. The valve is connected to the incoming
water supply, and is opened and closed by the lever which has the float mounted on the end. When the water level rises, the float rises with it; once it rises to a pre-set level, the mechanism forces the lever to close the valve and shut off the water flow. This is an example of
negative feedback and of
proportional control.