Babbage’s barrels

by Adrian Johnstone, Royal Holloway, University of London and Paul Curzon, Queen Mary University of London

Workings of a music box with bumps on a barrel flicking metal prongs
Image by Monika Schröder from Pixabay

Despite his hatred of Barrel organs, Babbage used barrels with relocatable pins in his machines. They worked in a similar way to a music box, where the pins flip the teeth of a metal comb to sound a note and by moving the pins you get a different tune. In Babbage’s version the barrel’s pins push levers that send information round the machine, determining what it does.

By programming the positions of the pins, different overall operations are created from the combinations of lever pushes. This is a similar idea to what later became called microcoding, in modern computers, where very simple low level instructions are used to program the operations available in a computer’s instruction set.

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This article was funded by UKRI, through Professor Ursula Martin’s grant EP/K040251/2 and grant EP/W033615/1.

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