Slide Rule ???

What is it?     Let's see encyclopedia.

• 1970's, all the actual production of slide rules ceased
  with the advent of electronic pocket calculators.
  They remain only as a memorial possession or collection of old engineers.
  
  
• Even though the actual history of slide rules has ended,
  small numbers of these are still being manufactured,
  and the competitions are held continuously even in the 21st century.
  
  
• Each of American slide rule manufacturers insisted that their rules were used on the space program,
  and that they made it possible for sending men to the moon.
  It is strange that Russia (Soviet Union) didn't make any good slide rules,
  seems that they competed in the space program with America
  using German slide rules. (Or Chinese made?)
  
  

Scales and characters


Top models of the most makers have around 24 typical scales:
2-C, CI, 2-D, DI, CF, CIF, DF, A, B, K, L,
S, T, ST, LL0-LL3, LL00-LL03.
Beyond these scales, the following scales are excellent, and added to be better.


• P (Pythagorean, sqrt(1-X²)) scale:
  It is quite difficult to read sine values for over 60° in C scale.
  If we read P scale with cosine value for over 50°,
  we can get quite precision sine value easily.
  Besides this, the P scale is very useful.
  
  
• The red complementary angles and the T1-T2 (extended tangent) scales:
  The complementary angle is easy to get from 90°,
  but it is not easy for the engineers who have too many things to think or calculate.
  At least, those make the engineers' life quite easier, and help engineers' hair to remain on the top.
  So does the extended tangent T1-T2 for over 45°.
  
  
• R1-R2 (Square Root) scales, and their L (Log) scale:
  The square values are to be read at A, or B scales,
  those are only a half length of the rule, and we feel something is missed, and uncomfortable.
  The square values from the R1-R2 really give us satisfaction.
  Furthermore the normal L scale is not precision enough at 25cm rule.
  The L scale based on R1-R2 gives us twice precision values than normal L scale.
  
  
• W1-W2, W1'-W2' (Square Root) scales:
  Those rules give an effect of double the length of the rule.
  It is a very good system, not a bad idea, but liable to err, and makes the life complicated.
  The results of calculations need to be checked again with the calculations of C-D scales.
  If I need more precision, I'd rather use a circular slide rule.
  
  

Double cursors in circular slide rule:

Once the simple theory of them is understood, it is not at all difficult to use.
However, the engineers started off using the normal slide rules at school,
became experienced, and did not like to change to the circular type.
21cm (8.25") of it has about twice precision compared to the 25cm (10") type rules.