The "bed of nails" device is the punched card reader. In the base is a pool of mercury. The card is "read" and tabulated when one of the nails falls through a punched hole in the card, into the pool of mercury below and completes an electrical circuit.

Designed by Herman Hollerith to compute the totals for the 1900 census using punched cards after getting his inspiration from a railroad conductor's ticket punching to identity passengers.