In response to Gene Wright's "Happy Square Root of 10 day!" (16 March) I wrote:

Quote:Stan Keeley and Martin Pinckney reported that the Hemmi 250 has the CF and DF scales folded at the square root of ten. I have a Hemmi 250 in my collection.

I am fairly sure that some slide rules were made with the CF scales folded at the square root of ten rather than at pi.

Page 38 of *SLIDE RULES - A Journey Through Three Centuries* by Dieter von Jezierski as translated by Rodger Shepherd states in part

Quote:The CF and DF scales on my K&E 4094 Merchant's Mannheim, K&E 4095 Merchant's Duplex, and K&E GP12 Business-Science slide rules all have the CF and DF scales folded at pi. In my collection only my A.W.Faber-Castell 67/22R Business slide rule does have the CF and DF scales folded at 3.6 .

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Folded scales ... ... were seldom incorporated in slide rules until 70 years later [about 1887] ... ... when Tscherepaschinski [a Russian] arranged a pair of scales folded at 3.16 along the lower slide-stator interface. Eventually, folded scales became common, especially in American duplex slide rules, which had four slide-stator interfaces. Some early German models had scales that were folded at the square root of 10, but most folded scales were folded at the value of pi. ... ... In the case of merchants' slide rules, the folded scales were folded at 3.6, thereby permitting direct multiplication or division by 360 days when carrying out calculations involving interest.

I can't recall needing the square root of ten in any of my calculations. Is there a technology where the square root of ten apprears frequently in calculations?