The operation of the tube was based on the fact that under certain operating conditions the voltage of a spot on the surface of the target plate would assume one of only twovalues. Which value it assumed could be controlled by altering the target supply voltage while a stream of electrons was directed at the spot. This tube has the advantage over other storage tubes of not requiring that the stored pattern be periodically refreshed. The tube was capable of storing 1024 separate spots- with a storage cycle time of about 25 microseconds. Whirlwind used two such tubes per digit to obtain a storage capacity of 2048 16-bit words.