Knowledge & Memory
The Slide Rule as a Logarithmic Arithmetic Engine
- Period and origin
- Seventeenth century onward, following Napier’s logarithms and Gunter’s scale; the central engineering computer until electronic calculators
- What it was built to do
- To multiply and divide by sliding, so that a hard operation becomes an easy one.
- How the mechanism works
- A positive quantity is represented by a position proportional to its logarithm. Sliding one scale by one operand’s position makes the other operand align at the sum of the two positions, which is the product. A cursor distributes one alignment across several scales.
- Input
- Two positive quantities, set as positions on logarithmic scales.
- Transformation
- Physical addition of lengths, which is multiplication of the quantities they encode.
- Output
- The product or quotient, read at the alignment.
- What is evidenced
- Established: the artefact or the documentary record carries this directly.
- What it demonstrates
- The most economical statement in the series: change the representation and the operation changes with it. Multiplication becomes translation.
Relationship to other mechanisms
- Next in the series
- The Pascaline and the Step Reckoner as Decimal Digital Arithmetic EnginesNext in the canonical series.
- Compares with
- The Sector as a Proportional Geometry EngineBoth compute by transferring lengths; the sector encodes ratio, the slide rule encodes logarithm.
Related article
Sources
- Owner monograph: 10_slide_rule_mathematical_monograph.md
Local copies of third-party material are records, not pages: what is published here is the citation and a paraphrase, never the document.