Selector switch, made by the Clark Automatic Telephone Company, United States, 1903. From automatic interdepartmental telephone switchboard installed at Guy's Hospital, February, 1903.
This is a selector switch comprising part of the internal telephone system used at Guy's Hospital between 1903 and 1932. It was a simple switch, which although not suitable for public exchanges, was useful for exchanges within a single building. Rotating the dial on a telephone to the number needed wound a spring inside the telephone, which drove a toothed wheel. Electrical impulses were sent to an electromagnet in the selector switch for each tooth that was passed, which then connected the telephone to the number called.