Full Details

Name and Model Number Spirule by Walter Evans
Country of Origin U.S.A.
Date of Manufacture  
Front Scales Specialised Scales
Back Scales  
Scale Length 29.2 cm
Type of Rule Other
Construction Plastic
Area of Use Other
Comment Used for addition of angles or multiplication of lengths of vectors.
The Spirule can also be used in plotting vectors or as a circular slide rule.
Rarity RR
Donor Collection of Rod Lovett
Match Number 27

RR = Rare -- 1 of 25 collectors has one; it may take a year to find an example in the marketplace.   
RRR = Very rare -- only very few known to collectors; may take several years before one comes up for sale.   
RRRR = Extremely rare -- a museum piece; unlikely that an example will ever come up for sale.