3479280 Telephone exchange system, 1850; (add.info.: New telephone exchange system repairs itself or calls for help. Printed circuit board: a technician inserts a printed circuit board into place in the new U.S telephone exchange system. Thousands of the boards are used in a central telephone exchange. Mounted on the boards are transistors, semiconductor diodes and other electronic components, and metal \'teeth\' on this end of the board make electrical connection with the rest of the equipment.); Universal History Archive/UIG; out of copyright.