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On 2016-09-26 23:48, Gary LaPook wrote:
> I had never noticed those gauge points. Looking at some of my slide rules I 
see that they have them except one K&E 4081-3 that has a "SRT" scale instead 
of an "ST" scale. I don't understand the difference between these two scales.

I'm sure many users were never aware of those little marks. Regarding ST
vs. SRT, I think the latter name came into vogue in the 1950s. This has
been hashed out in the Yahoo slide rule group. But even late in the
slide rule era many makers just called the scale ST. Functionally
there's no difference.

The scale is a compromise. It can't be optimum for all three functions.
Although the sine and tangent of a small angle are nearly the same as
its size in radians, the three values are not identical. This is most
evident at the right end of the scale:

.09585 = sin 5.5
.09599 = 5.5° in radians
.09629 = tan 5.5

The difference is visible if you carefully set 5.5°. My ca. 1957 K&E
4181 SRT scale clearly gives radians. Its sines will be a little high
and tangents low.

Unlike sines and tangents, the radian conversion function of ST/SRT can
be extended up and down by decades as you please. For instance, 1° on ST
aligns with .01745 radian on C or D. But that could just as easily be
.1° = .001745 radian or 10° = .1745 radian.

   

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