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The IBM 731 was formally announced on May 21, 1952. It was the intermediate storage device of
the 701 Data Processing System because it offered greater capacity than the cathode ray
tubes in the IBM 706 Electrostatic Storage Unit but it had a
slower access speed.
Each of the two drums in the Magnetic Drum Reader and Recorder stored 40,960 digits, for a
total capacity of 81,920 digits. The reading and writing heads, approximately 1/1,000th of
an inch from each drum surface, read and wrote data while the drum spun at 2,929
revolutions a minute. The average time for transmitting one word was 1.28 milliseconds.
The average random access time was 50 milliseconds. Each digit, recorded as a small
magnetized spot on the face of the drum, could be recalled in an average of 40/1,000th of
a second. Characters could be retained indefinitely in this unit as turning off the power
had no effect on the contents.
The 731, which rented for $1,400 a month, was withdrawn from marketing on October 10, 1954.
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