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Website last updated 03 August, 2010

CFM Scale

The CFM scale is a C scale folded at log(e). It's found on the slider, and its counterpart DFM is found on the stock.

ln(10) is about .43, which is not close to the middle of the C scale. Thus, although you can jump up to CFM, DFM when your result is offscale at C,D, there are better folded scales for this. Read Tutorial on Folded Scales here.

A better use for DFM is to read base 10 logs off of the LL_E scales. If you have LL_X scales, you can read their base 10 logs on D, but LL_E scales are in base e. DFM has the conversion to base 10 built in so you don't have to do an extra multiply.

If your LL_E scales are on the slider, you would include a CFM scale, if they're on the stock, include a DFM scale.

The result is merely the log base e value on D multiplied by .43. All the other considerations for finding logs and exponents with the LL scales apply.

In the slide rule image on this page, you can see that we're finding the log of 1000, setting the cursor at 1000 on LL3_E, and reading the result 3 on DFM. The CF/M,DF/M pair are the equivalent logarithm converters in the case of a slide rule with LL_X scales.

The scale names make it hard to remember which is which, but historically, that's what they were called. It doesn't help that this scale reference is a word for word reprint of the CF/M reference with the names and numbers changed! Luckily, you'll only find one set or the other on a given slide rule.