Caps threaded 1/8"-27 I.P.S. Fit standard lamp stems. Set screw prevents rotation of socket. 3-Way sockets operate dualfilament lamps on sequence of low, medium high and off. Aluminum brass dipped finish. 250 watts, 250 volts. Temperaturerating: -40 to 149 degrees F. UL Listed, file E7430, UL 496 requirements. NOM Certified.