This calculating device features a relatively straightforward working principle: 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. Also, if dragged clockwise back to the 0 positions, your number is added to the result.
The construction of the Add-A-Mite pocket calculator was not very successful since the small numbers make it difficult to read, and there is no fixing mechanism, not even at zero, so the setting cent is almost analogous and thus quite prone to error.