The Westinghouse WL‐759 ``trigger'' tube is designed to operate small relays and yet is controlled by currents less than 10−11amp. in the starting anode circuit. Investigation shows that this is possible because of an ``avalanche'' effect set off by the radioactivity of the cathode after the starting anode has attained the critical potential for the efficient development of the high ion density needed to bring about the complete development of a glow discharge between the cathode and the principal anode of the tube. Other electrical characteristics of the tube are also discussed.