Artifact Details


The First Magnetic Random Access Mass Memory with Interchangeable Media

Catalog Number





John T. Potter, who founded Potter Instrument Company in 1942, was a prolific inventor with over 80 US patents. On September 1, 1948 he applied for a patent on a "Three-Dimensional Selector and Memory Device", and it was issued as patent number 2,620,389 on December 2, 1952.

The invention was for a digital storage device that used steel wires strung on a set of two-dimensional frames that were packed together in rows. The machine would access one of the frames, then a particular wire in the frame, and then the data recorded somewhere on the wire. It was three-dimensional memory. In 1955 Potter negotiated a contract with the of the Univac military computer division of Sperry Rand to develop and build three of the three-dimensional memories. The Project Engineer was George Comstock, who began work on January 2, 1956.

Just sixteen months later, in April 1957, the three working completed units were delivered to Univac in St. Paul, Minnesota. But there were several significant differences from the invention as disclosed in the patent:

1. Instead of wires, the storage medium was strips of the 1⁄2" mylar-backed magnetic tape that was commonly being used in reel-to-reel computer tape drives.

2. Instead of using rotating shafts and electromagnets, the three-dimensional positioning was done using hydraulic cylinders with binary-coded displacements. This was a variation of a positioning system using binary- coded rotating rings that Potter had filed a patent on in 1951.

3. The storage bin containing all the tapes could be taken out of the unit and replaced with another, making the total amount of storage unlimited. This interchangeable media provided true removable data storage. None of the other random-access magnetic storage devices of the time, like IBM's RAMAC disk drive, had removable media.




Comstock, George E., Author
Shustek, Len, Author


Computer History Museum

Place of Publication

Mountain View, California


12 p.




Random access memory (RAM); Potter, John T.; Comstock, George; Interchangeable media


Gift of Len Shustek

Lot Number