Claude Shannon built a desk calculator that required its owner to brush up on Roman numerals. To multiply seven by eight, you entered VII on the main keyboard, then pressed a separate set of buttons in the order I, I, I, V. That was VIII backward, with a pause after each press while the machine worked. Shannon called his creation THROBAC, and the Roman numerals continued inside the machine, through the arithmetic itself.[1][2]

In the instructions he wrote on April 9, 1953, Shannon explained how to plug in the power supply and clear the machine before beginning. His opening description compared its controls to those of a Marchant desk calculator, giving this peculiar project the familiar treatment of an appliance with a manual. THROBAC could add, subtract, multiply and divide, provided its operator observed a few limits.[2]

The main display went only as high as LXXIX, or 79. Let a sum reach 95 and the result would appear as XV, fifteen, because the counter wrapped around at eighty. Division put the quotient on an upper display and the remainder below, but the upper display could show no more than ten. Shannon explained that limit by pointing to the number of positions on the stepping switch he had used.[2]

As the user entered Roman numerals, the calculator had to accommodate the way their letters change a number according to their order. In IV, the I must be subtracted; in VI, it must be added. Shannon arranged relays to read the entered symbols from right to left and apply the appropriate rules. He also allowed some flexibility at the keyboard: four could be entered as IV or IIII, with the machine presenting the result as IV. Multiplication proceeded by repeated addition, and division by repeated subtraction.[2]

Shannon had considerable experience finding uses for relays. His master's thesis had applied Boolean algebra to switching circuits, and his 1948 paper on communication laid the groundwork for information theory. Those achievements helped establish modern digital communication. With THROBAC, he had also worked his way through the practical details of making a desk machine calculate in I's, V's, X's and L's. The MIT Museum lists its full name as the Thrifty Roman Numeral Backwards-looking Computer.[1][2][4]

When John Horgan visited Shannon at home for a profile published in 1992, the calculator was among the gadgets in a room next to the one displaying his awards. Shannon moved quickly from the honors to the machines, where his own creations shared space with things he had collected. Horgan saw a mechanical mouse that could find its way through a maze and a juggling manikin modeled on W. C. Fields. Shannon told him that he had pursued his interests with little concern for their eventual practical value.[3]

For his visitor, Shannon tried to coax the W. C. Fields figure into a demonstration, but it would not perform. "I love building machines, but it's hard keeping them in repair," he said.[3]

Sources

  1. MIT Museum: THROBAC, archived December 7, 2024
  2. Claude E. Shannon: Throbac I, April 9, 1953, Collected Papers, pp. 695-698
  3. John Horgan: Claude Shannon, Tinkerer, Prankster, and Father of Information Theory; original profile published April 1992
  4. MIT News: Claude Shannon, Digital Communications Pioneer, Dead at 84

Image: Claude Shannon, unknown photographer, Tekniska museet, via Wikimedia Commons, CC BY 2.0. Cropped and placed on a neutral background. Archival portrait, not a photograph of THROBAC.