Experiment 9
Superconductivity

General Introduction

I'm sure there are good sections in your textbook talking about superconductors and their present and future uses -- so I'm not putting in a "theoretical ramblings" section for this lab, because of lack of time. However, you should probably have a look at the sections on what's important and what things to watch out for; both from a theoretical point of view and from a "how do we get this done right/quickly/at-all" point of view, they should hopefully help.

What's Important in This Lab

Hints and Tips for making this lab a better experience

• Make sure to record the uncertainty associated with each type of measurement, along with some note about what gave rise to that uncertainty. You'll need it for doing the data analysis, and you need it to be complete...
• The lab manual available on the web has some slight typos, but they're just misspellings and won't affect the way you do the lab, so just be patient with Raf (one of the TAs for 114, who's had to update both the experiment and the manual during this semester, while teaching the other P114 labs and taking some actual classes himself).

Part I (of the lab): Levitation

• No, we're not doing a David Copperfield show here; this is real levitation!
• The lab manual says to put the LN (short for Liquid Nitrogen) into the styrofoam tray (or, for the second part, the Petri dish) first, then put in the superconductor. However, it then says to make sure that the liquid level should be right up to the top of the superconductor. Now, if you didn't put in enough LN then you can add some more, but if you put in too much then you'll just have to sit and wait. So, start with just a little bit, and add more if needed.
  Why is it important for the LN level to be even with the top of the superconductor? It's not critical, it's just that you'll get the best results that way. You see, if the level is too low then the top of the superconductor will tend to heat up and may cross its Tc and stop superconducting. On the other hand, if the level is too high, then the magnets that you'll be putting on top of the superconductors will sit in the liquid rather than being suspended in the air! This will give them much more resistance to moving than just air resistance, so they won't be able to spin for as long, and also for the second part it'll be hard to see how far above the superconductor the magnetic ring is...

Part II: Critical Temperature

• If you read the old version of the manual for this lab, then it talks about using a multimeter to get a voltage reading and translating from that to temperature using some table given in the back of the lab manual -- forget all that. We have devices to hook to the thermocouple leads that read out directly in Kelvins, so you can just read the temperatures straight off. Much easier.
• Do this part a few times, until it looks like your data is pretty well grouped and repeatable (i.e., you don't have a huge standard dev.).

Part III: Resistance vs. Temperature

• See my comment in Part II above about the fact that we're reading the temperatures directly now and not using some silly lookup table.
• The lab manual seems pretty clear about the steps in this section. Just remember that each reading you take down should have three numbers:
  1. voltage across the superconductor
  2. temperature of the superconductor
  3. time (since you started taking readings)
• Make sure to handle the superconductors with the wires very carefully! Specifically, don't pull on the wires at all, as they can come off from the superconductor pretty easily, and if they do while you're doing the lab then that's it...

Things to Watch Out For (safety-type issues)

• Liquid nitrogen is extremely cold (77K, which is -196 degrees C or -321 degrees F). This is not stuff that's safe to go wild around. Yes, it's fun, but if you've seen demos where they dip a rose in and then shatter it against the table, remember that the same thing happens to your fingers if you try it. Enough said; just be careful.
• The superconductor discs are ceramic, and are pretty fragile, so handle them with care. Also, they are made of strange metal oxides and could very well be toxic (read that: we don't really know yet, and I don't really want you to be the guinea pigs), so don't handle them with your bare hands.
• The superconductors with the leads hooked onto them are especially delicate assemblies, as I mentioned above. Be really careful with these.
• The lab manual says to stop running current through the disc when it gets to about 130K or so. Definitely turn off the current before it gets up to 200K. The basic problem is that at room temperature or anything close to it, these little discs are not only not superconductors, but (like most ceramics) they are insulators, and if you have this power supply still trying to pump a constant current through it, it'll generate a lot of heat and ruin at least the disc and possibly the power supply as well...
• Another note from the lab manual: when you're done with each disc, warm it carefully as it says in the manual, with a distant hair dryer for just a couple of minutes, to dry it off. Moisture left on the ceramic discs corrodes and degrades them.