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Booth's Magnetic Drum Storage
The real problem Booth had to solve was storage.
He worked his way through the possibilities and ended up thinking that magnet storage was the only really practical system. He tried a matrix memory not that far from Forester's core memory, but he couldn't make it work.
Then he discovered the "Brush Mail-a-Voice" recorder. This used a 10-inch paper magnetic disk to send a low quality recording of a voice through the mail. Booth took the disk and spun it at 3,000 rpm. The idea was that spinning at that speed it would be rigid and this would allow him to float a head over the surface using the Bernoulli effect.
Yes this was the first floppy disk, or was it the first hard disk, or perhaps the first Bernoulli drive!
In any case it doesn't matter much because it didn't work. The disk was unstable and it "flapped" so much that the head couldn't get anywhere near it and eventually it disintegrated. As Booth commented
"I suppose I really invented the floppy disc, it was a real flop!".
After the floppy failure he turned his attention to a drum. A 2-inch diameter brass cylinder was plated with Nickel and spun. An array of 22 heads read and wrote 21 tracks of data and a single clock track. The whole device gave 256 words of 20 bits and acted as the memory for the ARC which was completed and working in 1948.
After further development the ARC had an electromagnetic store for 50 numbers and a pluggable sequence controller that could hold a program of 300 instructions.
After the ARC Booth turned his mind to electronic machines. The ARC's control logic was originally designed using valves and one of his graduate students built a small test electronic machine - the SEC (Simple Electronic Computer). This lead on to the APE(X)C All-purpose Electronic (X) Computer where X could be replaced by the name of any sponsor who came up with the money to build one!
The first was the British Rayon Research Association and so the APE(Rayon) Computer was built.
This used a redesigned magnetic drum built by Booth's father’s company Wharf Engineering. This company produced magnetic drums for use by the new computer industry and many were exported to the USA. The first model used 415 valves and was huge. Later the British Tabulating Company (later to become ICL) became interested in his designs and derived their HEC (Holerith Electronic Computer) from his APE(X)C project.
All BTC computers were called something starting with "Holerith" because they started out producing punched card equipment. A prototype HEC was shown in 1953 and eventually built under the new name of BTM 1200. Five were sold. A slightly upgraded model with a 1024-word drum was produced in 1956 and 70 of these sold.