Clackson scroll formula

The Clackson scroll formula,
:<math>l = \pi \cdot s \cdot n^2</math>
is used in blacksmithing for estimating the length <math>l</math> of stock required to produce a scroll of <math>n</math> turns with a (on-center) spacing <math>s</math> between the turns.
Derivation
We can think of the scroll as having a thickness of s and being wound tightly so that there is no space between the layers. When wound up the scroll is roughly a cylinder with a radius of s·n, so its cross-sectional area is roughly πr πs n . On the other hand, the scroll's cross-sectional area whether wound up or not is l·s. Thus we have ls πs n , or l = πsn .
 
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