A 1928 Punch-Card Patent Switched From Round Holes to Oblong Ones So Closely Spaced Columns Wouldn't Tear

The cards were read electrically. A brush rode over the card as it moved, and wherever there was a hole it touched the contact underneath and closed a circuit. The patent notes that for that to work, “a certain length of time is required.”

That’s the trap with simply making round holes smaller to fit more of them. A smaller circle gives the brush less time over the hole. Stretch the hole along the direction the card travels, and the brush stays in contact longer, so “a greater length of time will have transpired than when circular holes” are used.

The second problem was the paper. Put round holes close together across a row and the card weakens like a perforated ticket, because “the circular perforations converge naturally to a tearing line.” Oblong holes with straight sides hold up better, offering “considerable resistance to tearing by a series of such perforations.”

With narrow oblong holes, “the spacing of adjacent card columns is reduced.” The patent spells out the payoff: “80 columns of data may be placed on a card compared to a 45 column card.”

It also offers a second way to spend the gain. Keep the old amount of data, and the new card “will be about half the size of regular Hollerith cards.”

The patent was granted on August 12, 1930. Its argument is a reminder that storage density often runs into a physical limit before a logical one: here, how long a brush needs to touch metal and how much paper has to stay between holes. What the patent leaves open is which choice the company would make: the denser card, or the same data on a card half the size.