Mechanical Calculator ➔ Speedee Add-A-Matic

The Speedee was a manual adding machine that could not perform subtraction.  It consisted of eight columns and five rows of keys in both maroon and ivory.  It had a nine digit display that could add up to 9⋅999⋅999⋅99.  It could perform multiplication by using repeat action.  In other words, if you wanted multiply 3 x 5 you would simply hit the number 3 five times or the number 5 three times.  The large maroon button on the left is the reset button.

If you examine the keyboard you’ll note that it only shows numbers 1 through 5.  If you want to use numbers 6 through 9 then what you do is hit two keys that add up to that number.  For example if you wanted to add 8 you would hit the 5 and 3 keys or the 4 key twice.  While this may seem odd to modern eyes there was a rational reason for this.  It was shown that professional operators could move very quickly on the bottom five numbers but would have to slow down to reposition their hands for higher numbers.  It was found (and yes, there were actual studies) that it was faster to simply hit two buttons and not have to reposition your hands.    Of course, it helped that this also made the mechanics less complicated and lowered manufacturing costs.