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A ballcock (also balltap or float valve) is a mechanism
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 Joseph Bramah and Thomas Twyford.The diagrams and description ONLY apply to the USA (these are not typical in the UK or Europe). It consists of a valve connected to a hollow sealed float (literally, a device that floats on top of water) by means of a lever, mounted near the top of the tank. 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; when the level rises to a pre-set level (called a fill line), 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.
When a toilet's handle is turned, a flapper valve is pulled upward by means of a rod or chain. It is designed to sink more slowly than the water, which will exit to the toilet bowl below, so that the tank may empty. As the tank water level drops, the float descends and actuates the fill valve. Water is fed to the tank to replenish its supply, and a smaller flow is directed into the overflow tube to refill the bowl.
Once the flapper valve closes, the water flow from the fill valve continues until the tank level again reaches the fill line. Should the float or valve fail and allow the water level to exceed the fill line, the water will pour into the overflow tube and out to the bowl. (Note that although this wastes water, it is a successful attempt to mitigate the flooding that would otherwise occur under that single-point valve failure.)
These devices are sold at most hardware, plumbing supply, and home improvement stores.