Dear Physics 24 students,

yesterday in lecture I went through a geometrical construction
needed to calculate the magnetic field of a solenoid.

Talking to people after class I realized that many of you were
confused. So I wrote it up carefully for you to look at.
I actually did it in three somewhat different ways.
You can find it here

http://hep.ucsb.edu/people/claudio/ph24-18/geometry.pdf

It is also done in the Purcell book on page 300,
Figure 6.16.  I don't think it is explained very
well (if at all) there, but maybe you disagree.

C.