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Invented by Charles H. Webb in 1868 for use in accounting offices, the device consists of two numerator wheels enclosed in an opaque case. The unit and ten wheel has two semicircular series of numbers, each from 0 to 99 inclusive; and the hundred and thousand wheel is numbered from 0 to 49 inclusive. The inventor proclaimed its simplicity and ability to carry its own tens and hundreds, and the Adder was offered for sale in contemporary advertisements at $6.00 (brass), $8.00 (with steel cam and stop) or $10.00 (steel plated throughout).
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