Conical Primary Length Formula

Here's the diagram and equation I mentioned, which gives the length of a conical spiral...the graphic and equation was kindly provided by Godfrey Loudner of the Tesla Coil Mailing List.  If the picture shows up smaller than it should be, enlarge it to full size--many of the newer browsers automatically shrink pictures that are "too large."  In this case, if the picture looks unclear (it's jaggy looking and the text is hard to read), then it's been shrunk by your browser, and you'll need to enlarge it to be able to understand it easily.  Here's his original post on the Tesla Coil Mailing List, which has a text version of the graphic below.  He explains his derivation here, and in a later post he corrected one thing: the helix case is not given by setting P=90; rather, that leads to an indeterminate form.  He also gives the formula for a helix in that post.