The lamp assembly illuminates the punched paper tape to throw an enlarged image onto the photo cells which read the holes in the paper tape. The smaller motor pulley drives the paper tape to 5,000 characters per second past the optical read aperture. ( 30 miles an hour!! ).The tension on the tape is set by the sliding wheel. It has to be checked regularly. The length of the tape changes with temperature and humidity. The WRNS who operated Colossus in the War, dreaded the tape breaking. There is a 1940s light switch on the frame in the centre of the picture. This switches on the paper tape reader, just like it did in the War.