Add-A-Mite Pocket Calculator circa 1950
Monogram of California
2500 18th St.
This simple mechanical pocket adder is made of aluminum with bright blue paint and dates perhaps from the 1950s. The cursor on the right is depressed and dragged to the number you wish to add, 0 to 99. When released, the cursor engages a gear tooth on the back of the disc and when you then drag the cusor clockwise back to the 0 position, your number is added to the result. Crossing from 99 to 00 causes an automatic carry into the hundreds window, allowing sums up to 2,499 (see the video below). Subtraction works similarly by starting from zero and dragging counter clockwise to the desired number.
Here is the back.
Here is a video of the adder in action (note: audio is delayed several seconds)