Fun with Magnets

So today I made a magnet, MacGyver stylee. It consists mainly of paper tape and wire 🙂 I decided to use Cu wire rather than a superconducting wire as I can probably get enough field from a normal magnet. It’s all very makeshift, I just need a magnet for the next fridge run. I could have got a superconducting one made professionally by the workshop with a tuffnell former, superconduting heat switch, etc., but it probably would have taken 2 weeks or something. If the mock-up works I suppose I’ll get a proper one made for next time. My old superconducting magnet (which worked fine) was “removed” because the new fridge didn’t fit down the centre bore. Which was slightly irritating.

MacGyver magnet also fits neatly inside my mu-metal shield, although I need to do some field measurements to check the attenuation level that the mu-metal is providing (It’s tricky to calculate that). Mu metal essentially attenuates nearby fields by ‘sucking in’ magnetic flux. The purpose of the shield is to attenuate the earth’s magnetic field and to reduce magnetic noise from nearby equipment such as VDUs (does anyone even use the acronym VDU anymore?), power supplies, mechanical pumps etc. The downside is that it eats any desired fields too. Most people ‘nest’ at least three mu-metal/cryoperm/superconducting shields to get a good low noise environment inside the cryostat. I’ve only got one at the moment so it will have to do until I order some more.

Here’s a piccy of the mag on the vacuum can, the mu-metal shield is sitting next to it.


I’ve made 500 turns already on the MacGyver magnet, and I’m considering adding another 500.

Assuming solenoid limit, B/I~mu0*N/L, so if I apply 100mA, B~4mT/A. So 100mA gives 0.4mT.

My old superconducting magnet calibration revealed that you need about 0.37mT to get phi0 in the junction (to traverse the Fraunhofer period), so this should be OK.

The wire is about 3.5Ohm per 40turns, so 500 turns will be 45Ohm. But the resitance will probably be 10 times less in the Liquid Helium, so 100mA into 4.5Ohms gives 45mW dissipation, which should be OK.

If 500 turns doesn’t sound like very many, well, it is 🙂 To put it in perspective, I got through 2 Delirium albums and a Blutengel album so far (I tend to listen to music whilst in the lab doing boring repetitive tasks.)

It didn’t help with my RSI.

Listening to Blutengel (a German vampiric ‘EBM-goth-eurosynth’ band) is also a bad idea – it makes me want to dress up and go bite people. Yum 😀


5 thoughts on “Fun with Magnets

  1. quantummoxie says:

    does anyone even use the acronym VDU anymore?

    Nope. Of course, I don’t know what it stands for so there could be a reason I never use it…

  2. rrtucci says:

    VDU is shorthand for Voodoo. That’s why it causes unnatural magnetic noise. When you work at the extreme limits of temperature and vacuum, like Susan does, you’ll try anything that has even the remotest possibility of helping.

  3. quantummoxie says:

    Ah, is VDU in that same sort of category as FUBAR and SNAFU – old military acronym?

  4. physicsandcake says:

    I believe it stands for Visual Display Unit. I think people started using CRT instead but they’re all but obsolete now anyway.

  5. quantummoxie says:

    See, I’m so gullible at times that you could get me to believe just about anything.

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