The front of the rebuilt bombe.
These were used to help decrypt messages generated by the Enigma machine. Each drum performs the equivalent function of one of the rotors in an Enigma machine, hence the grouping in threes (one for each of the rotors in a three-rotor Enigma). The bombe could perform the same task as thirty-six Enigmas running in parallel, and was driven by an electric motor to quickly step through all of the different rotor positions.