Keying of S Scale
The S (sine) scale is
treated in two substantially different ways on slide rules. In the “Rietz”
arrangement of
trig scales (S, S&T,
T), angles between 0º-34’ and 5º-44’ are set on the S&T scale
and the value of the sine of
the angle is read
from the C/D scales with the knowledge that it lies in the range 0.01 to
0.1--e.g., sin(4o)
=0.0697. Larger
angles, in the range 5 degrees 44 minutes to 90 degrees, are set on the
S scale and the
value of their sines read
from the same C/D scales as before but this time with the knowledge that
the value
lies in the range 0.1
to 1. In other words,the angle 4o is the same distance from the end
of the S&T scale as
its sine, 0.0697, is from
the end of the C and D scales and the angle 40o is the same distance from
the end of
the S scale as its sine,
0.643, is from the end of the same C and D scales. This is true for all
rules that have
an S&T scale.
There is no S&T scale
in the “Mannheim” arrangement (S, L, T) of trig scales; all angles from
0 degrees 34
minutes to 90 degrees
are set on the S scale and the value of their sines (range: 0.01 to 1.0)
are read from the
A/B scales.
Thus, there are two different
ways to set up the sine scale: Rietz (keyed to C/D) and Mannheim (keyed
to
A/B). Slide rules
with [S, L, T] scales usually use the Mannheim sine scale but several Hemmi
slide rules,
made between WWII and
1955, with [S, L, T] scales used the Rietz sine scale. After 1955
these rules switched
back to "Mannheim" sine
scales. (Got that? Some Hemmi Mannheim rules used "Rietz" sine
scales.) The
catalogue notes which
style is used on closed body rules with [S, L, T] on the back of the slide.