Otis King Model K
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Otis King's Patent Calculator

Cylindrical Pocket Slide Rule with Helical Scales, Made in England by Carbic Ltd., London 1921-1972

Many thanks to Andrew Davie and his Slide Rule Trading Post for helping to launch this little site.

Otis King Model K

This popular model, shown the to left, does multiplication and division, and not much else.

Model K is characterized by scale 414 on the bottom and scale 423 on top. See Flat Otis King's Scale 423.

Otis King Model L

The model L adds a linearly divided scale section on the top, instead of two decades of logarithmically divided scale. A linear scale on a slide rule is used to find logs and anti-logs, which can be used along with multiplication and division to do arbitrary powers and roots.

The bottom scale for model L is usually scale 429, with 430 on top, but other pairs of scale numbers are sometimes found on older units. See both scales unrolled by scanning.

Table of Otis King's patent calculators

I used to maintain the most complete table of known Otis King's Calculators thanks to inputs from the world's collectors, and from you SRTP visitors. It now lives on its own page, instead of here. The data in the table helps us form and test theories of how to date these items from their serial numbers; see my plea for more data below.

See also my summary table already updated from the one published in the Journal of the Oughtred Society.

In recent years I've continued to gather over 100 new entries per year, but I haven't gotten around to updating these tables since about 1999.

Finding "Mr. King"

Otis King's "roots" have recently been uncovered by his half-great-grand-niece, who has contributed her story, with his family tree and birth record.

British Patent 183,723

Take a look at Otis Carter Formby King's patent drawings and claims.

U. S. Patent 1,645,009

Colin Barnes recently discovered that Otis King also got a U.S. patent.

Otis King N2150Detail

Scales and Images

See my scale detail page for accuracy info and detailed views of the scales.

The scan at the right is a detail of the lower scale, between the "handle" (below) and the "cursor" (above) on Otis King's model K serial number N2150. This unit appears to be all brass, like some that are found, but is actually just severely worn. You can see the black paint remaining around the cursor screw (screw is found on "Type B" units) and the Nickel or Chrome plating remaining in the corner of the handle. The detail shows the wrapped varnished paper scale clearly. Picture made on Apple Color OneScanner by ScanWizard (me--self portrait on scanner).

Classification into Types

See my OK types page for information on variations that were made over the years, and their classification into types A, B, C, D and variants.

Instruction Sheets

I have a copy of a new and unusual variant of the Otis King's instruction leaflet, contributed by Jim Stewart, from his model K number Z5702. This one is a 6-page trifold, saying "The Otis King Calculator is Distributed in the U.S. by / Calculator Company / Post Office Box 593 / Lakewood, California 90714". It shares the picture and almost all of the text with the British versions, but omits the section "Sterling Calculations". Page 1 with the address is pictured.

Andries de Man has published HTML'ized versions of the modern 16-page instruction leaflet, the older 8-page instruction leaflet, and the very old (1922 or so) 12-page instruction leaflet for Otis King's Patent Calculator (models K and L), and also an Otis King Slide Rule ad brochure.

We also know of 3-page and 4-page versions, and stapled and unnumbered variants of the usual 8-page folded version. In addition, the GENIAC Pocket Calculator, an Otis King relabeled in New York by the Oliver Garfield Co. has its own modified version of the 8-page folded leaflet (just the name and address on the front are changed, but they may have actually printed the scales as well, since they don't say Carbic or Otis King anywhere on some; also, some units don't say Made in England anywhere).

OKs also sometimes have a paper or plastic box insert card with decimal equivalents for old shillings and pence, and for fractions of an inch. See images of a two-sided insert and an unusual wooden box.

Some other old brochures and pamphlets have recently been brought to my attention, and I will endeavor to get them online. Bug me if they don't show up soon.

A 1923 Otis King sales letter

Read this letter from Carbic's sales manager, dated 26th September 1923, which was found in the original cardboard shipping box with model K serial number 9201, along with the wooden coffin-style box with latch and both 3-page and 4-page instruction leaflets. Letter copy contributed by Bob Otnes.

More info needed

If you have or see an Otis King's, please send me more info:
Serial Number:  letter and 3 or 4 digits, or rarely just digits
Type: A, B, or C.  see OK types page, or JOS vol. 4 no. 2,
  or tell me where it says "Made in England" and
  whether the cursor has a screw or a dimple
Model: K has one double-length log scale on top, 
       L has two different scale sections on top (log and linear)
Scale numbers:  414 & 423 for model K 
                429 & 430 for model L (or 416 & 417 on old ones)
                424 & 422 for model N 
Scale color:  black-on-white or inverse
Scale material:  varnished paper, plastic coated paper, or 
                 paper with plastic film (I'm not sure how you tell)
Handle Plating:  Shiny Chrome vs. Dull Nickel vs. unplated Brass
Cursor surface:  Painted Black vs. Brass vs. Nickel vs. Chrome
Address:  from instruction sheet or elsewhere
Date:  written on instruction sheet or box, or from memory of purchaser
  (dates are what we're really after, but be careful getting them)
Instructions: old 12-page, middle 8-page, new 16-page, or other.
  if 16 pages, does it imply decimal Sterling on page 8 "Money Calculations"?
    old Sterling (pre 1971):
      The Sterling items must be reckoned as decimals of pounds, 
      shillings or pence as best suits the problem
    vs. new decimal Sterling (1971+):
      In Sterling calculations 1/2 new pence is set as 005. Thus 
      the setting for 22 1/2 np is 225. For 3.01 1/2 np the setting is 
      3015; do not forget the '0' in such amounts.
   [don't just say "Sterling version", since that's all of them]

Dick Lyon dicklyon@acm.org

(650) 941-4263 home or (650) 253-3128 work

OK Serial Numbers