Quinn and Ittner's (1963) superconductivity experiment

[beowulf.geata]

Homework Statement

The problem is Exercise 1 here: http://www.open.edu/openlearn/science-maths-technology/engineering-and-technology/engineering/superconductivity/content-section-2.1.

I am interested in question (c), where you are asked to estimate the maximum possible resistivity.

Homework Equations

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.

 

[NascentOxygen]

Imagine you took a 17mm length of old-fashioned water pipe. It would be made of lead. That's why plumbers are so-named, because they worked with Pb, from the Latin, plumbum.

A straight length of round pipe can provide us with a circular path for current flow. Look at the pipe from the end, there you see the circle for the circular path.

To make the pipe fit into some narrow gap we have for it, you squash it almost flat, but not totally flat. We don't want the top of the circle to be squahed so hard that it contacts the bottom. We want to still keep that circular path, though by now it is resemblng a square path. To stop the top from making electrical contact with the bottom, you could insert a SiO₂ spacer into the tube before we squash it.

The "ring" in the diagram is not drawn to scale; thngs might be clearer if it were. But on looking at the dimensions on the figure, you can estimate the minimum path length for current flowing around that squashed pipe.

I hope that helps.

 

[beowulf.geata]

Many thanks. That was very helpful. I can see now that I was confused by the fact that the figure is not drawn to scale.

(Just for the sake of completeness: I made a mistake with the date of the experiment, which dates back to 1962, not 1963!)