Caps threaded 1/8 in -27 I.P.S. 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 watts, 250 volts. Temperature rating: -40 to149° F. UL Listed, file E7430, UL 496 requirements. NOM Certified.