The Friden had two registers on the carriage and one on the main body in the multiply section. It had a negative (but no positive) multiply button and a great many other buttons and switches. The more modern Monroematic CSA-10 pictured below had three registers on the carriage with keys to enter numbers into (or clear) the appropriate register. (The black paddles below the keyboard on the right shifted the carriage left and right - the same function as the crank to the left.) Multiplies were done by explicit repeated additions (the + and - keys repeated if held and the "R" key prevented the keyboard from being cleared.)Ī multiply/divide switch on the upper right corner of the keyboard controlled the direction of the counter and the two metal levers on the right side of the carriage cleared the upper and lower dials. It had a divide lever but no multiplication key.
The older Monroe model 1 shown below was electric but still had many manual devices including an exposed operating crank and a manual carriage position crank (on the front left of the machine.) This machine's operation was quite obvious compared to the machines below. (Especially useful when an errant operator asked the machine to divide by zero.) Monroe Divides involved successive subtractions that could take a long time, so DIVIDE STOP was a common key.
(Although they no longer added as you pressed the digits and had zero or blank keys to fix mistakes.) The accumulator/displays of these machines were mounted on typewriter-like carriages that moved back and forth during multiplies and divides. The numeric input keys were still usually arranged as a set of digits per column like the Comptometer. (Hand driven machines continued to sell well however because they were quieter, lighter, smaller and less expensive.) While the Comptometer made addition practical and fast, it was these motor driven models that did the same for multiplication and division. Motor driven mechanical models first appeared in the 1900's and had become commonplace on the desks of engineers by the 1940's.