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The paper tape moves between the light source and the electronic sensors. The speed of the tape establishes the speed of the computer. The signal derived from the sprocket hole is the clock pulse that drives the logic circuits of the entire computer. The Colossus tape reader was typically run at 5000 characters per second. For computer users today, 5 kHz may seem to be a slow computer execution speed, but that left only 200 microseconds for the logic to fully process each Nazi message character. The tape moves at the rate of 41.7 feet per second -- that's a high speed tape reader! And it was built that fast in 1943! War time memories say that as an experiment the tape reader was sped up to 9000 character per second. The computer logic worked fine at that speed but the tape broke and threw tape around the room at a rate of 50 miles per hour. So it was normally run at about 5000 characters per second to reduce the risk of paper cuts to the people in the room.