Otis King's Patent Calculator Model L
Otis King's Patent Calculator Model L Z0169, circa 1921
Cylindrical Slide Rule with Helical Scales 429 and 430
What at first appears to be a modest spyscope is in fact a remarkably compact cylindrical slide rule patented in 1921. This little gem is known as Otis King's Patent Calculator and can perform multiplication and division, logs, anti-logs, square roots and arbitrary exponential powers.
The compact helical scale is effectively much longer than a standard slide rule, providing greater precision than would ordinarily be possible -- the model K's scale is the equivalent of a 66 inch slide rule!
Below is a thumbnail of a fantastic walking stick mounted with an Otis King Slide Rule:
Walking Stick mounted with Otis King's Patent Calculator
The photos (from the 2003 East Coast Oughtred Society Meeting) can be seen here.
Otis King Patent Calculator Model K with cursor in brass
Otis King's Calculator Patent Abstract GB183723