Full Size Original
Each Hollerith tabulator was equipped with a card reading station. The manually-operated card reader consisted of two hinged plates operated by a lever (similar to a waffle iron). Clerks opened the reader and positioned a punched card between the plates. Upon closing the plates, spring-loaded metal pins in the upper plate passed through the punched data holes in the cards, through the bottom plate, and into wells of mercury beneath. Pins that passed through the punch card completed an electrical circuit when contacting the mercury below. The completed circuit energized the magnetic dials on the Hollerith tabulator and advanced the counting hands. Upon completion of the electrical circuit (signaled by the ringing of a bell), the clerk transcribed the data indicated by the dial hands.