#### Dave's slide rules

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# Dave's slide rules

I just made it into the last generation of slide rule users: studying chemical engineering in 1973 we used slide rules routinely for calculating. Four-function electronic calculators were around but expensive. By the end of the year the TI30, HP35 and a few other scientific calculators were available and affordable, and slide rules disappeared overnight.

My modest collection includes a few examples of the different types of slide rules that were once so useful to science and engineering. I've grouped them into three major types - linear, circular and cylindrical, and a few other odds and ends. I'm interested in comparing the accuracy and usability for multiplying with different types and sizes of slide rule. Have a look at my world record slide rule lower down. Dave Hoyer, 2012.

# Accuray and precision of slide rules

Some thoughts on the meaning of the terms accuracy and precision, and how to determine the number of significant digits of a slide rule...  Here

# Linear slide rules

Linear slide rules were by far the most common for everyday scientific and engineering calculations: straight, with slider and cursor. Some have multiple sliders.

Scale lengths 36 to 500 mm (1.25 to 20"). Significant digits: 2.2 to 4.0

# Circular slide rules

Circular slide rules use two dimensions (a flat circle) and are often more compact than linear slide rules of similar accuracy. Some have the scale spread around multiple turns in order to achieve a longer scale and hence greater accuracy. The multiple turns can be either concentric rings like the Fowlers long scale, or a continuous spiral like the Ross Precision Computer.

• Single turn - Scale lengths 100 to 630 mm (4 to 25"). Significant digits: 3 to 4
• Concentric - Scale lengths 1 to 3 m (3 to 10'). Significant digits: 4
• Spiral - Scale lengths 1 to 20 m (3 to 60'). Significant digits: 4 to 5

# Cylindrical slide rules

Cylindrical slide rules use three dimensions to achieve high precision and accuracy with ease of use. The scale is arranged either as a helix around a cylinder like the Fuller's, or as lengthways sections along and around the cylinder, like the Thachers and Loga. They are bulkier than circular slide rules of the same accuracy, but easier to use.

• Helical - Scale lengths 1.6 to 15 m (66" to 66'). Significant digits: 3.8 to 5
• Segmented - Scale lengths 1.7 to 20 m (66" to 66'). Significant digits: 3.8 to 5

# Eximius Diu 6

Here is my attempt at a large, practical and accurate slide rule. It's a flat spiral slide rule 750 mm diameter, with a 175 metre long log scale comprising 135 turns of the spiral. It is equivalent in accuracy to a linear slide rule 318 metres long, and accurate to 6 digits over its entire range. As far as I know this is more accurate, and substantially longer, than any previous slide rule.