The resulting sum appears in the rightmost column of the central operating area. A single digit is exposed on the side of each sliding bar (when viewed from an angle), and these taken together form the answer. In the photo above, the result is 987,654,321. The result may also be inferred from any viewing angle with a glance at the center-point of each split-colored slider.
The adder was intended to be placed on the ledger, and manipulated from above, as shown in this advertisement. However the the operator must tilt his head down - or tilt the bottom edge of the adder up - in order to make out the result-- not very convenient! Also, my finger easily slips off the knob as I'm sliding a bar, and it is difficult to recover the calculation once this occurs.
The operating instructions helpfully admonish us to: "Remember that the adder can make no mistakes. If you ever get an incorrect result it will be because you have failed to operate it properly."
This adder is a favorite of mine for its colorful eccentricity, but all in all, it's not hard to understand why it is relatively rare -- more than a few were likely tossed onto the trash heap!
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The Locke Adder
The red finish shown here is rare (source ebay)
Save Your Brain
"Cannot make mistakes" and "Saves time, labor, brain" (source ebay)
Below is a 'less than mint' condition Locke shown diassembled
Fowler's Adding Machine 1863
An influential progenitor to the Lock Adder
(source ebay)
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