John Napier's Binary Chessboard Calculator

Dedication

Author Sid Kolpas dedicates this article to the memory of his co-author, Erwin Tomash (1921-2012), "a great mentor and friend".

Introduction

John Napier (1550-1617) is best known for his invention of logarithms and of 'Napier's rods', the latter a set of marked sticks designed to simplify multiplication in the pre-calculator age. Among Napier's less popular and lesser known computational aids was his 'chessboard calculator', a device based on the binary number system. After a quick review of Napier's logarithms, 'rods', and 'promptuary', this article describes Napier's 'chessboard calculator', including how it can be used to perform the four operations of arithmetic as well as root extraction.