how do puch card readers work?

how do punch card readers work?

kelseymh8 years ago
http://www.google.com/search?hl=en&q=how+do+punch+card+readers+work&btnG=Google+Search&aq=f&oq=
Search result #9, paragraph 6, "Electric wires in the card reader made contact with mercury in a cup through holes in the cards to complete an electric circuit and "read" the data."