I end up buying a lot of weird things for my lab-- really expensive sand, for example-- but the latest purchase was a little strange even by my usual standards:
The other day, on my way into work, I stopped by the store and bought a roll of parchment paper, for use in the lab. It actually makes perfect sense, though it'll require a little explanation, below the fold
The brass thing in the picture above is a length of 3" diameter pipe, which is intended for use as a holder for a magnetic field coil. The coil, one of a pair used to produce a magnetic trap for neutral atoms, will consist of roughly 500 turns of copper wire wrapped around the brass pipe, and the whole assembly will be slipped over one arm of the vacuum chamber.
There are a couple of technical issues involved with this. First, there's going to be a fairly substantial amount of current pushed through these coils-- several amps-- so they'll generate a fair bit of heat. That's the reason for thec opper tubing soldered to the ends of the brass pipe-- we'll run cold water through that, the keep the coild from overheating.
Another issue is that we'd like to be able to switch the current through the coils on and off fairly quickly. I've got a couple of nice fast-switching current supplies for this purpose, but the speed is limited by the number of turns, and the presence of the brass pipe. The more coils you have, the harder it is to switch the current quickly, and it gets even harder if you have a continuous ring of conducting material inside the coil, thanks to Lenz's Law: the changing magnetic field generated by the wire induces a current in the brass pipe that resists the change in the field, making the switching go more slowly.
We can't do anything about the wire itself (or the steel vacuum chamber), but we can get rid of the problem introduced by the pipe, by simply cutting through one of the walls, so it's no longer a continuous loop. Removing one band-saw-blade worth of brass reduced the switching time by better than an order of magnitude, with a small test coil.
Of course, you need to make sure that when you wrap the wire around, you don't squeeze the pipe back together to make a loop again, which is where the parchment paper comes in: I wrapped the pipe in parchment paper, slipping one end of the paper through the gap in the pipe, to provide an insulating gap in the pipe. It also has the bonus effect of keeping the wire in the coil from short-circuiting to the brass form.
When we did this back at NIST, we had some Teflon-coated paper that they carried in the stockroom there, but I'm on a tighter budget than they were, and cooking parchment seems like a pretty good replacement. It's silicone-impregnated paper, so it's waterproof and electrically insulating, and it's normally used for baking (Kate swears by it for making cookies), so it can handle the heat load of the magnetic field coils.
How do we make the actual coils? That's a Shop Day, and I'll talk about it in another post. Here's a teaser picture of a (mostly) finished coil, though:
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Do you have any trouble getting such purchases past the beancounters, or do you just pay for it out of your pocket? For example, nail polish of different colors is useful in electronics, to fix the position of variable capacitors, pots, etc., and to mark components. But try buying nail polish with contract funds. I knew a guy at the Naval Research Lab who did it once, as a stunt. Wasn't worth it. (Dollar stores to the rescue -- they have really cheap nail polish in really garish colors.)
As a grad student, I bought some spectroscopic-grade distilled water and took a spectrum of it. The spectrum had tons of formic acid features. The boiling point of formic acid is 100.8C, so it's practically impossible to get rid of by distillation. Examination of the fine print showed that the supplier dealt with the problem by allowing formic acid in their definition of spectroscopic-grade water.
So I went to the Big Bear supermarket and bought a plastic gallon jug of unknown-grade distilled water. The spectrum had no identifiable features except water. Put out a tech report on the spectrum of water from 400 to 4000 cm-1.
Do you have any trouble getting such purchases past the beancounters, or do you just pay for it out of your pocket?
The parchment paper was something like $2, so I just paid for it myself. I've occasionally bought things on the credit card, and then reimbursed myself, but it's always a hassle.
For example, nail polish of different colors is useful in electronics, to fix the position of variable capacitors, pots, etc., and to mark components.
That's another thing I need to do at some point, but I keep being thwarted by random macho pride-- I don't want to be seen buying nail polish. Especially really garish nail polish at the Dollar Store...
If anyone out there is baking cookies *without* using parchment paper, they must not know the trick yet. The difference is very noticeable.
Dammit, now I want cookies...
I seem unable to purchase every item on my grocery list, so I occasionally end up baking cookies without parchment paper. The advantages of parchment paper I've noticed are that it is easier to get the cookies evenly baked, the cookie sheets are easier to clean. and the cookies are not so oily. However I've never had a need to prevent electrical current from being conducted from the cookie sheet to the cookies, so I'm not sure what this has to do with the topic of the article ...
Parchment paper is great for baking. Also cool is silicone mats. Nothing (so far, at least) sticks to those things.
By the way. Magnets are cool.