With the increasing use of thyristors in electric installations such as lamp controllers, electric fan controllers, and electric heat controllers, thyristor interference in the middle frequency (f < 20 MHz) range is repugnant and seriously hampers the use of thyristors. A physical model of thyristors indicates that interference results from the instantaneous pulse that is produced when the thyristor is turned on. Such interference will produce an interference pulse on a nearby antenna that is received and amplified to become annoying noise. This conclusion is useful in the design of electric installations using thyristors so that such installations have greater reliability and better interference prevention.