Editor’s Note: Arthur S. Jensen (1917-2012) received a BS in Physics (1938), MS in Physics (1939), and a PhD in Physics (1941) from the University of Pennsylvania. During WWII he was a Naval Reserve Officer on the faculty of the U.S. Naval Academy teaching physics and physics of aviation until March of 1946. Following the war, he became an engineering research physicist at RCA Laboratories in Princeton, New Jersey.
At this time, there was an active search for a random access electronic computer storage device. In England, the Williams-Kilburn tube was patented in December of 1946. The Selectron tube was being developed by RCA for John von Neumann of the Institute of Advanced Study at Princeton University also in 1946. The William-Kilburn tube was used for the program storage of the Manchester Baby, which became the first stored program computer in 1948.
Here is Arthur’s story:
Eureka!
Just before New Years 1946, still on Navy terminal leave, I reported as a physicist to RCA Laboratories in Princeton, New Jersey. It was a dream job.
Dr. Vladimir K. Zworykin, the inventor of the iconoscope and the orthicon, the first television camera imaging sensors, was the manager of technology. Dr. Albert Rose, the inventor of the image orthicon, the standard studio television camera imaging sensor, was the manager to whom I was to report. And finally, Dr. Paul Weimar, the inventor of the Vidicon, the small television imaging sensor that enabled the making of a handheld, portable television camera, was on the staff. How wonderful to be permitted to learn from and work with such stars!
I was assigned to make a vacuum tube that would store one line of a radar signal and compare it with the next line to detect moving targets. The application was secret, but not much was made of it those days; I was simply told not to mention it to anyone except the government project manager, who visited from Fort Monmouth to hear how I was progressing. The device was a vacuum tube because this was before the invention of the transistor.
A lot of people during those days were trying to make a single device that would replace the many individual double triodes that were used for computer memory: one double triode in a flip-flop circuit to store one bit of data, eight such to store one byte! Each tube had a heater to activate its cathode. So many, in fact, they required a large air conditioner. Computers were too large, filling an entire gymnasium!
In England, several were trying to store lumps of electrons on the faceplate of an oscilloscope tube. Dr. Jan Reichman at RCA Labs was trying to assemble 64 double triodes into one large single vacuum tube, not so much of a heater, but a beast to build.
I knew very little about vacuum tubes, electron optics, or computers. Fortunately, my daily half hour with Dr. Rose was most educational. Essentially, he tutored me on all these subjects, and he was very patient with me and my progress. The project had been attempted and remnants existed, an electron beam tube that resembled a Vidicon but a mica sheet target replaced the optically sensitive surface. Two electrical engineers had built the electronics necessary to operate it, but it simply failed to
show anything on an oscilloscope except where the electron beam hits the edges of the target. I conferred with these engineers, the engineer in charge of constructing whatever was described to him, and Dr. Weimar.
Then one day Dr. Rose, describing some effects in electron optics, mentioned the words, “coplanar grid effect.” Afterward, I mulled over these words that I had never heard before. I looked at the screen that was placed over the mica sheet to prevent redistribution of the secondary electrons that would quickly erase any stored electrons; this was the bane of the English engineers. The screen was specially made to be very thin. I drew diagrams of the cross section of the mica sheet and the screen. I drew equipotentials representing the electric field. It took a few days, but I realized that the screen I had was too thin. It had to be thicker to prevent coplanar grid effect, to shield one lump of electrons stored on the mica from its neighbors.
I found out from the engineer charged with constructing the tubes what screens he had. One was a woven screen, somewhat courser than what I had been using, but whatever. He made a tube with a woven screen. Voila, it worked. We could store a line of charge, compare it with the next, and observe the difference! When I showed it to the government project manager, his remarks were, “Fine, but why that magnetic field with that massive field coil? Can’t you make it electrostatic?”
Of the electrostatic oscilloscopes available from RCA Lancaster, Pennsylvania (which manufactured electron tubes), the electron gun of one looked just right. I drew a new diagram and showed it to the engineer in tube construction. Easily obtained, easily constructed, done. The electronic engineers easily revamped the electronics. Voila, it worked. The government project manager was pleased. Soon RCA Lancaster was manufacturing the tubes for the government. Shortly after, engineers from several places were visiting me to learn how to use the Radechon, the Barrier Grid Storage Tube; in those days all special electron tubes had names.
One, Dr. Jack Harrington of MIT Lincoln Labs, realized that he could store 64 x 64 lumps of electrons on the mica surface, 4,096 bits = 512 bytes of information. Thus, I had invented the first compact random access computer memory! Three patents resulted: 2503949 (April 23, 1949), 2538836 (October 15,1948), and on the electrostatic version 2598919 (June 30, 1950). RCA was in the business of licensing patents so 23 companies manufactured the Radechon.
Because the Radechon was a small package having only one heater for 4,096 bits, it was used for over 10 years in many computers. Dr. Harrington used the Radechon in radar image analysis computers on the DEW line across Canada. The Bell Telephone System constructed the first all-electronic switching office, in Marion, Ohio. Even the Soviets copied that tube, making samples that were much larger and more carefully machined, even using the name “Radechon” in a Soviet show in New York City in about 1955. Dr. Malcolm Gager at the Naval Research Laboratory realized he could store a TV type picture on the Radechon, enabling him to invent computer-aided tomography (CAT scan), but for radar imaging; the use of computer-aided tomography was not applied to x-radiation until 10 years later, after solid-state data storage was invented.
Arthur S. Jensen, IEEE Life Fellow
Parkville, MD
