Post Reply 
WP34s Not My Grampa's RPN Calculator
02-10-2015, 05:19 PM
Post: #1
WP34s Not My Grampa's RPN Calculator
When I was a 17 year-old kid (the same year the HP35 was introduced) my Grampa gave me his Comptometer; ca. 1929. Grampa was the comptroller for Kansas City in the mid 20th century, and he used this Comptometer in that job, and on into his private tax practice into the seventies. Yes, its RPN (explained later) which implements decimal ring counters, multiple simultaneous carries, 9's complement subtraction, and eight full digits with carry|overflow and error correction.

[Image: comptometer.jpg]

The accumulator (register x) is a series of ratcheted decimal ring counters capable of multiple simultaneous carries; which operated in RPN mode long before Jan Ɓukasiewicz did his work in the 1950's. On the Comptometer the accumulator wheels held the first addend and the second addend was entered on the down-stroke of the entry push-buttons... the post-fix addition operation was carried out on the up-stroke of the entry buttons. Unlike the ten-key adding machine, the Comptometer's buttons were pressed by a trained operator in 'parallel' all-at-once, rather than serial one-by-one. The display wheels are aluminum drums, very thin, and very light... which helped give this machine its speed, even for a mechanical device.

[Image: accumulator.jpg]

The (clear x) function is performed by the silver level mounted through the right case bulkhead... you can see the error correction button (red) in the background. The clear rotated the accumulator wheels and gears outward and away on a pivot (believe it or not the accumulator had its own serial number) allowing the wheels to spring back into rest position, ready for the next set of additions (or subtractions). The silver lever served no other purpose than to clear x.

[Image: clear_x.jpg]

This boiler plate was standard on Comptometers of the period, and sported the dates of the various patents applied for over the years... you can see that his J model comp indicates the first patent in 1887, and the last (prior to its manufacturing date) in 1920.

[Image: boiler_plate.jpg]

Felt & Tarrant Mfg Co of Chicago Illinois produced these machines for many decades, which were used in the United States and the U.K. well into the 1970's in back offices and schools. If your high school still taught the slide rule in its analysis and science classes more than likely the high school business classes still taught the Comptometer! This J model was manufactured in 1929 with serial number 2194.

[Image: serial_no.jpg]

The case work of these beautiful copper 'shoebox' adding machines is unmatched for manufacturing quality, durability, and beauty. i am lucky that this unit has all of its original parts including case corner screws. This 86 year-old RPN calculator is in perfect working order and other than a few blemishes (less than most of us will have at that age) it has not changed since the days when my grandfather used it in a Kansas City back office in the early to mid twentieth century.

[Image: case_work.jpg]

I have restored two of these machines as a general interest and hobby. By scavenging parts from four beat-up and rusty Comptometers I was able to restore two of them to full working order with all parts including error correction and overflow. The mechanical engineering in these wonderful calculators is a marvel. I often wonder what will exist of our calculator collections one hundred years from now!


Kind regards,
Find all posts by this user
Quote this message in a reply
Post Reply 

Messages In This Thread
WP34s Not My Grampa's RPN Calculator - MarkHaysHarris777 - 02-10-2015 05:19 PM

User(s) browsing this thread: 1 Guest(s)