INJECTOR, an induced current apparatus which by means of a jet of steam feeds water to a boiler. The invention of Henri Giffard, its introduction in 1859 created great interest, because it seemed impossible that a jet of steam from a boiler could force itself and the water back into the boiler. Yet not only this was done, but later researches proved that exhaust steam at atmos pheric pressure could be utilized, and it is possible now to deliver against a pressure of I 50 lb. per sq. in. The Davies and Metcalfe exhaust injector will do this, and with the help of a small supple mentary jet of live steam it will feed against a pressure of 25o lb. per sq. in. Exhaust steam has no velocity in the atmosphere, but if issued into a vacuum it acquires a very high velocity; hence by condensing the steam by contact with the feed water such a vacuum is produced, the velocity of the steam, imparted to the water, giving sufficient force to carry the latter into the boiler. (See also EJECTOR ; STEAM GENERATION.)