9.9 Rule Change 4092-3

See the talk presented at the Oughtred Society virtual meeting, May 15, 2021.

Also, the article in the Jour. Oughtred Soc., V30.2p63 (2021),

and its corresponding Letter to the Editor in the Jour. Oughtred Soc., V31.1p7 (2022).

After writing the previous vignette about Enrico Fermi and his personal slide rule, I obtained a renewed interest in the history of the development of this particular model. Over several years I happened to collect four Keuffel and Esser model 4092-3 slide rules from the Chicago region. In March 2021 I was working on my slide rule database, and while scanning through a list of scales on each rule, I noticed that in the listing one of the 4092-3’s had its scales in a slightly different order than the other three. At first I thought it must be a typographical error from my data entry efforts, but when I looked at the four rules again, indeed one was different in a significant way.

The outlier in my possession has serial number 64405, and a quick look at the McCoy web site for K&E slide rules suggested a date of about 1923. Further investigating from this site reveals that the scale set for the 4092-3 changed in 1925. From 1922, when the rule was introduced, until 1925 the 4092-3 did not have a K scale (Scale Set #1). This changed in 1925 when the K scale was added to the top of the rule (Scale Set #2) and this scale set lasted until this model was ended in about 1937. (My other three 4092-3 slide rules contain Scale Set #2.) Looking at the online photos of McCoy’s collection, the K&E labeling and patent numbers appear to have been shifted from the top of the rule (Scale Set #1) to the bottom of the rule (Scale Set #2) at this time. Examples can be found in images of Scale Set #1, s/n 13495 and Scale Set #2, s/n 261497 on the McCoy web site.

The interesting feature of my particular slide rule with s/n 64405 is the fact that this “1923” 4092-3 has a K scale at the bottom of the rule’s front face. The K&E labels and patent numbers are found at the top of the rule as in Scale Set #1. And the K scale barely fits onto the rule as a series of tick marks along the bottom, while there is plenty of white space left at the top. This suggests to me that this may have been a trial-run or pre-production rule constructed during the transition period between scale sets.

I can just imagine someone saying, “Let’s see if the K scale will fit on the bottom, then we won’t have to change anything else.” And the result, shown above, might then have led to the final changes we saw in Scale Set #2.

When I purchased the slide rule it came with a K&E sewed leather case typical of the 1920s, and evidently had two separate owners during its useful lifetime. One owner was apparently Doc McClain, of Joliet, Illinois. This person not only etched this name into the case, but also etched the date “1925” in two places, as well as “JJC,” presumably for “Joliet Junior College” (America’s first community college, formally accredited in 1917). Since the date on the case is consistent with the date of the switch-over between scale sets, I assume Doc is the original owner. A separate owner, “Haddie Langdon, Notre Dame,” is etched on the case as well.

While it could have been a “test case” for the new scale arrangement, or part of a pre-production trial run, or less likely a manufacturing error, it is interesting that it was sold or even made available to the general public. In any event, this 4092-3 with serial number 64405 rule has a new place in the Collection as one of those “rare finds.”

K&E 4092-3 Transition Rule, s/n 64405.

The original 4092, introduced by K&E in 1909, was the first slide rule offered with a segmented set of Log Log scales for use in calculations of exponential growth, compound interest, and other computations involving numbers raised to various powers. The 4092-3 and 4092-5 variants were introduced in 1922 and last offered in about 1937, giving way to newer models with Log Log scales. Since the discovery in the Collection of the transitional version, other early model 4092-3’s have been obtained. The following image composed of these rules nicely illustrates the evolution of the model during the roaring 20’s:

Evolution of the K&E 4092 Front Scales.

NOTE: Following publication of an article on the slide rule in the Journal of the Oughtred Society in 2021, two new pieces of information have surfaced. First, a Letter to the Editor of the Oughtred Society was received from Maynard Wright, who revealed the existance of his 4092-3 with serial number 64950 and which does not have the new scale set with a K scale. This lends credence to the possibility that our rule 64405 is perhaps the result of a test run or a very limited run for this particular scale set. Second, Fred Astren noted during the 28th International Meeting of Collectors and Researchers of Slide Rules and other Historic Computing Instruments, September 17, 2022, that he has in his possession a 4092-5 – serial number 62906 – which also has the K scale at the bottom of the rule. This would indicate that up to approximately 1500 such rules may have been produced (if all rules between these two were indeed model 4092s). However, a counter-example exists that shows that not all rules between these serial numbers had the lower K scale. The example in our Collection has serial number 64163, which does not have a K scale at all. (See the image below.)

  • Fred’s 4092-5 —- 62906 — lower K scale
  • Mike’s 4092-3 —- 64163 — no K scale
  • Mike’s 4092-3 —- 64405 — lower K scale
  • Maynard’s 4092-3 -— 64950 — no K scale
  • Mike’s 4092-3 -— 131217 — upper K scale


Below are shown the various K&E 4092 slide rules in the Collection today:

Model Year Serial Style
4092 1913 original scale set, LL1,2,3; no K scale, serif type
4092-3 1922 6742 early scale set, LL0, no K scale, serif type
4092-3 1923 64163 early scale set, LL0, no K scale, serif type
4092-3 1923 64405 rare transitional rule, with K scale at bottom, serif type
4092-3 1925 131217 new scale set with K scale at top, serif type
4092-3 1929 277653 with K scale at top, sans serif type
4092-3 1929 295472 with K scale at top, sans serif type
4092-3 1933 441408 with K scale at top, sans serif type
N4092-5 1936 547636 with K scale at top, sans serif type

See also the vignette Fermi’s Golden (Slide) Rule.