Caps threaded 1/8 in - 27 IPS. Fit standard lamp stems. Set screw prevents rotation of socket. 3-Way sockets operate dual filament lamps on sequence of low, medium high and off. Aluminum brass dipped finish. 250 W, 250 V. Temperature rating of -40 to 149 deg F. UL listed, file E7430, UL 496 requirements. NOM certified.