Length of magnetic path?

I find the formula for the Reluctance of an air core solenoid as;

Reluctance = (length / cross-section) x (1/mu)

In the description length is just the length of the solenoid. Why wouldn't the length be equal to

2 x the length plus some distance around both ends? This would seem to be the distance the lines of flux travel.


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This assumes an infinitely long coil where the length is normalized unity length or some other convienient number that is a small fraction of the total. Per meter or per cm is usual. Indeed, in a short coil with defined ends, the path is much more complicated and there are emperical formulas to deal with it.

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Bob Eld

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