Abstract: A foam proportioning inductor apparatus is mounted atop a supply tank for providing a quantity of fire extinguishing foam concentrate. The nozzle has a through passage for the flow of water, as in an automatic fire extinguishing system, and has a supply line branch extending into the supply tank for pressurizing the tank with water and driving out a foam concentrate through a concentrate outflow line extending from the supply tank and into the inductor nozzle downstream of a constriction. The combination of the water charge in the tank and the venturi effect created by the nozzle constriction draws the foam concentrate into the water flow stream. The concentrate outflow line has a removable and replaceable pipe section containing a selected size of orifice, permitting the pipe section to be readily changed to select different sizes of orifices to correctly proportion the foam concentrate outflow relative to the different types of foam used in contempory automatic fire fighting systems.