1. The problem statement, all variables and given/known data The problem is Exercise 1 here: http://www.open.edu/openlearn/scien...neering/superconductivity/content-section-2.1. I am interested in question (c), where you are asked to estimate the maximum possible resistivity. 2. Relevant equations 3. The attempt at a solution I am afraid I don't get their explanation about what counts as "l" and "A" in the formula for resistivity ("For the tube, the length of the current path [...] is essentially twice the width of the silicon oxide layer, and the cross-sectional area perpendicular to current flow [...] is the length of the tube times the thickness of the lead films."). I think the problem I have is that I can't quite visualise the whole situation. I was unable to find a copy of the original paper on the internet so I would be very grateful if someone could shed some light on this for me. Thank you.