How do I demagnetize my radiometer?

For longer than I can remember we've had a Crookes radiometer sitting on
our kitchen windowsill where it heralds the coming of spring by starting
to turn when the morning sunshines on it through the window.
If you're not familiar with what I'm describing, see this:
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This morning I noticed that the radiomether was illuminated, but wasn't
doing its thing. I nudged it and it started turning but stopped in a few
I gave it a closer look and saw that the metal "hub" the vanes are
attached to was "stuck to" the side of the bearing needle, and it looks
like it's magnetism and not an out of balance condition because it
seems it will "stick" anywhere around its periphery.
I'm guessing the hub and the needle are both made of steel and somehow
got magnetized.
I don't own a demagnetizer big enough to fit the whole bulb in, and
before I spend the time winding a BF coil and energizing it through a
transformer and variac, does anyone have any ideas what else I could do
or use to attempt to demagnetize this old family friend? (Without
busting the glass that is.)
Reply to
Jeff Wisnia
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Only thing I can guess is that maybe someone shook the gadget and the parts rattled against each other. I'm remembering how as a kid holding an iron rod in your hand and hitting the end with a hammer would shake up the atoms so they lined up with the earth's magnetic field and the rod became a weak magnet.
Jeff (Well at least I had an answer...)
Reply to
Jeff Wisnia
Any old-school TV repairman will have a degaussing coil about a foot in diameter and would probably do the job for free.
Reply to
Jim Stewart
Couldn't you just stick a loop of coat-hanger wire in place of the tip on an electric soldering iron... and pass it over the globe? I wouldn't think it's magnetized as strongly rare-earth magnets or anything. LOL David
Reply to
David Courtney
Or you could use an electric motor with the rotor removed as your coil.
Reply to
I wouldn't think it is ferrous for just the reason that you described. Probably aluminum, copper, brass, paper, plastic, etc. The black will still absorb heat for the unbalance ref. the white.
Reply to
I guess if this were mine I'd do the following:
1) wind a ten-or-so turn coil out of bare number 12 or 10 house wire. Space the turns out so they don't touch. Coil to be just large enough to slip over the radiometer bulb.
2) connect that coil to the largest weller solder gun you own.
3) pull the trigger on the gun when it is far away from the bulb.
4) *Slowly* advance the coil towards the radiometer. The vanes inside may start jumping around and could possibly break the glass. Do this step slow and abort if it starts to get too exciting.
5) pass the back and forth over the bulb, don't let go of the trigger.
6) bring the bulb and the gun apart, to full arms' length before releasing the trigger.
Don't use insulated wire, the coil will get *hot*. Don't let the coil touch the glass bulb for the same reason....
Reply to
jim rozen
Wrong - lost the vacuum in the bottle. Likely a slow leak and maybe when the internals got hot it went through the seal. Rather common.
If you think it is magnetized - the vanes are Al or Mg - for light weight. But if you want - pass it through a loop that is passing AC.
One of those is the color TV - remember - when you turn on the TV there is a Twang sound - that is the coil getting a shot of juice upon power. Keeps the tube set.
Just put the unit on a wood table or such - at the center of the screen and turn on the set.
Most won't let you do it more than once for a while... Just once or twice is needed over time.
Martin Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net NRA LOH & Endowment Member NRA Second Amendment Task Force Charter Founder IHMSA and NRA Metallic Silhouette maker & member
Jeff Wisnia wrote:
Reply to
Martin H. Eastburn
Plenty of demagnetizing advice by others. I would use a turn of #12 copper wire in my soldering gun. It is very common for steel to become weakly magnetized from the earth's field when left in one position for a long time. Don Young
Reply to
Don Young
But if that were the case, why would it run for 15 to 30 seconds when exposed to sunlight and then get "stuck" when the rotating part wobbles a bit so the "hub" touches the vertical needle and sticks to it as though it were magnetized?
I don't doubt the vanes are made from a lightweight material but it looks like they are attached to a small metal hub stuck on the bottom of the glass nipple resting on the top of the needle. It's that hub which "sticks" against the needle when it stalls out.
I understand that these radiometers work better when there's a very low pressure in them than they do at a hard vacuum. Something to do with having the right amount of air molecules inside which get heated by the dark side of the vanes and give that side a bit of a kick.
I know I can buy a new radiometer for less than $12, but my curious mind wants to see if demagnetizing would "fix" this one.
But if you
I'll probably make a coil or pull a scrap motor apart over the weekend and see whether I can win this one with demagnitization.
Reply to
Jeff Wisnia
The one we had at home as a kid had a glass hub.
If you have a metal one - did the needle eat it up and finally cut it ? Have you turned it upside down and such ?
Martin Martin Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net NRA LOH & Endowment Member NRA Second Amendment Task Force Charter Founder IHMSA and NRA Metallic Silhouette maker & member
Jeff Wisnia wrote:
Reply to
Martin H. Eastburn
That's right. Those things actually run 'backwards.'
If it were momentum transfer going on, the white side of the vanes would be trailing, because the photons are reflected off that side, and absorbed on the dark side.
Instead the dark side gets a tad hotter, and the gas molecules in the envelope get a slightly stronger kick near the black surface, so the black side of the vane trails.
Reply to
jim rozen
The "hub" looks like an inverted subminiature glass test tube about 5/8" long and 1/8" ID. A little stamped metal X shaped piece with an extruded center hole is either press fitted or adhesively attached to its open end. The paddles are attached to the legs of the X.
My radiometer only has two paddles which are attached to opposite legs of the X. The other two legs are bare, maybe the manufacturer uses them when they make four paddle models.
What I prolly should have done before I started this thread was to slowly move a strong magnet towards the radiometer while it was stopped to see if it caused any movement or tilting of the vane assembly. That prolly would have told me whether my assumption that magnetization is making the metal hub "stick" to the needle has any legs to it.
Dumb of me not to think of that, as right in the next room there's a pretty strong suspension magnet on the little diamagnetic demo I built a few years ago.
If you've never played with diamagnetics, it's free floating objects right before your eyes, sort of counterintuitive to what we usually think about magnets and/or gravity:
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I'll run a magnet test first before I try demagnetizing with my 250 watt Weller as suggested.
Report later.....
Thanks guys,
Reply to
Jeff Wisnia
Well, when I got home tonight the first thing I did was take a high energy magnet and move it toward the radiometer, which was in dim light and not turning.
The magnet had no effect on the vane assembly, it didn't tilt, wiggle or twitch even when the magnet was right against the glass bulb.
So much for my thinking it was magnetism causing the metal X piece of the hub to stick to the needle.
Looking at the hub assembly under magnification, the metal X piece looks like it's likely made of aluminum.
But, the "sticking" effect was still there, With the rotor well centered on the needle, if I tilt the bulb slightly so the hole in the metal X piece stars moving off center with respect to the needle it soon reaches a point where the rotor will suddenly "jump" the rest of the way so the hole in the X piece hits the needle with an audible click and sticks there even when I return the unit to an upright position. I have to jiggle it until the rotor ends up centered asgain.
I'm thinking now it's probably an electrostatic effect, but I don't know what may have happened to make it to start happening now. I'm pretty sure It didn't do that a dozen or so years ago when it was new.
I tried wiping the outside of the bulb with some antistatic fluid we use on the glass of our photocopier, but that didn't help.
I guess nothing lasts forever.....
Reply to
Jeff Wisnia
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The magnet would equally magnetize the hub and the needle (If both were not similarly magnetizable, you would not be having this problem in the first place.)
I think that your first assumption that it was magnetized was correct, and that your test with the permanent magnet proves or disproves nothing -- it give the behavior which I would have expected.
Looking at mine, it looks as though it was stamped out of cheap tin-plated steel stock (tin can material), with the four arms twisted 90 degrees bent at right angles, stuck through slots in the corners of the flags, and then bent over to clamp the flags in place. Yes, it could be aluminum, but it looks more like tin can material, and that would be cheaper at the time this was manufactured.
However -- mine shows no reaction to a rather strong magnet, so it must be aluminum in my case at least.
The bearing is a combination of a tiny glass test tube (sorta) and the upright needle.
FWIW -- mine has a very cheap black plastic base vacuum-formed from thin plastic. Just barely enough to support the globe.
So -- still try the degaussing -- with a coil from a soldering gun or various other things which have been suggested.
Enjoy, DoN.
Reply to
DoN. Nichols
Hey! Has anyone suggested that it may be a result of "deforming" the tip of the needle or the bearing that rests on it???? Some sort of a "ring" worn in the bearing or some "unspecified" deformation of the needle. Some more examination please. ...lew...
Reply to
Lew Hartswick
Doubt it lew, read on....
First, after I posted last night I tried a three turn coil of #12 wire in my 250 watt Weller gun carefully moved over the bulb while it was on and removed several feet away before I released the gun's trigger. It did squat about removing the "sticking" effect.
So, the radiometer and I went to sleep for the night and when we woke up early this morning we were BOTH back to normal. It was overcast outside at dawn and there wasn't enough sunlight coming in through the window to move the radiometer.
But, to my angst and amazement it no longer displayed that "sticking" effect. I could slowly tilt it to from side to side and the rotor stayed horizontal until the needle reached the limits of the hole in the X shaped metal piece. No sudden "jumping" effect anymore.
Poiting a flashlight at it made it start turning.
I'm now strongly leaning toward it being an electrostatic effect and the charge(s) must have leaked off in the dark overnight.
Maybe there's a photoelectric effect from the coatings on the vanes which makes the rotor assembly charge up after light hits them for a while. The "light" sides of the vanes look like they're covered with a light yellow paint and the "dark" sides look like they're matte black paint.
'Tis interesting to say the least. I've posted to sci.physics to see if the effect my radiometer displayed is perhaps known and understood by some of the cognicenti there. Thus far all I've received on that newsgroup are a couple of useless remarks made by people who sound like their entire alimentary tracts have somehow become reversed.
Reply to
Jeff Wisnia
Isn't that one of Murphy's laws?
Reply to
Tom Miller

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