The Float Switch: A Liquid Level Turned Into a Contact

The float valve shuts off a cistern by itself. A float switch does the same job for an electric circuit: a float rides the liquid, and at a set level it opens or closes a contact — to start a sump pump, stop a tank overfilling, or warn that something has run dry. George J. Papashvili of Los Angeles applied it to the most expensive thing to run dry. US 1,719,078, *Float Switch*, filed 25 May 1926 and granted 2 July 1929, connects a cylinder to the engine's crankcase so the oil stands at the same level in both. A float on a spring-held rod drops as the oil falls; first it lights a lamp behind a glass marked "No oil", and then, if the driver drives on, it shorts the ignition and stops the engine. This rung works out a float's lift and why most float switches do not rely on it directly, sizes a sump pump's switching band, and builds a two-level float switch.