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Coil Inductance Without and With Water in the Center

05/07/2014 5:23 AM

How great, if any, change there may by in a coil inductance of 0.1 mH (3 cm long, some 50 turns wound around 5 cm diameter empty plastic pipe) when water enters the pipe ?

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#1

Re: coil inductance without and with water in the center

05/07/2014 5:37 AM

It will alter by something approaching the ratio of the permeability of water divided by the permeability of air. The exact figure can be determined locally by experiment.

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#2

Re: coil inductance without and with water in the center

05/07/2014 7:59 AM

Relative permeabilities (compared with vacuum):

Water = 0.999992

Air = 1.00000037

I think it would decrease by 837 pH with water instead of air. It would be really hard to measure, I suspect.

http://en.wikipedia.org/wiki/Permeability_(electromagnetism)

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#3

Re: coil inductance without and with water in the center

05/07/2014 9:05 AM

...so as a technique for detecting the difference between the presence or the absence of water in this particular pipe, the technique is unlikely to prove workable.

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#4
In reply to #3

Re: coil inductance without and with water in the center

05/07/2014 9:42 AM

Exactly. A sound based sensor would be much more practical.

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#5
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Re: coil inductance without and with water in the center

05/07/2014 9:47 AM

Indeed. There are dozens of proven ways of doing it:

  • Refractive-index-based sensors are widely used throughout the water supply industry in the UK for leak detection in double-containment piping systems.
  • Tuning-fork-based liquid sensors across a range of industries
  • Conductivity-based sensors
  • Etc., etc...
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#6

Re: Coil Inductance Without and With Water in the Center

05/07/2014 10:13 AM

Clean city water? Rusty well water? Coolant from a machine tool station?

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#7

Re: Coil Inductance Without and With Water in the Center

05/07/2014 10:23 AM

Seawater? Sewage? Heavy water?

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#8

Re: Coil Inductance Without and With Water in the Center

05/07/2014 10:27 AM

The inductance change wil be minimal but the Q factor of the coil will deteriorate drastically (and more as the water has more disolved impurities) and this could be easily detected. S.M.

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#19
In reply to #8

Re: Coil Inductance Without and With Water in the Center

05/08/2014 12:27 AM

GA!

Depending on the frequency the losses could be significant.

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#9

Re: Coil Inductance Without and With Water in the Center

05/07/2014 11:01 AM

Thank you !

So, I'm not going to buy an induction meter now (which I still do not possess) because it would be useless.

The liquid is antifreeze - some concentration of propylene glycol with water.

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#60
In reply to #9

Re: Coil Inductance Without and With Water in the Center

05/12/2014 5:08 AM

So not water now? A heat transfer fluid instead? Well, that's different. The permeability of heat transfer fluid can be obtained by contacting its manufacturer directly. Any inductor will still be subject to the same considerations as in #1 above.

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#10

Re: Coil Inductance Without and With Water in the Center

05/07/2014 12:38 PM

The presence of water will have a negligible effect on the coil inductance. Water is very slightly diamagnetic and will tend to reduce the inductance by slight amount. The greater effect of the water will be one of increasing the interwinding capacitance.

Water has a high dielectric constant compared to most other common materials. Whereas the dielectric constants of glass, plastics and so forth fall pretty much in the range 2-10, the dielectric constant of water is much higher, around 81.

The extent of the water's influence on the interwinding capacitance depends in large part on the thickness of the pipe wall and the interwinding distance. 50 turns in 3 cm suggests a closely-wound coil - or not, depending on wire gauge and insulation thickness.

Now, if the pipe wall and interwinding spacing are comparable to the wire diameter, the presence of water will measurably increase the interwinding capacitance. By incorporating the coil in, say, a resonant LC circuit, the increased capacitance from the water will lower the circuit's natural resonant frequency.

A related approach would be to short-circuit the coil and measure the change in its self-resonant frequency when water is present. Here, nearly ALL of the circuit capacitance is between the windings and so any change in the dielectric constant will have a much more pronounced effect on the resonant frequency than if an external C were used. In this setup you would typically incorporate a second coil consisting of only a few turns, wound adjacent to the first, creating a resonant transformer of sorts which you could then use as the basis for an oscillator.

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#11
In reply to #10

Re: Coil Inductance Without and With Water in the Center

05/07/2014 3:42 PM

Errata: I meant to write "...open-circuit the coil...", but on second thought, better to place a small amount of capacitance in parallel with the coil. Silver-mica capacitors are best as these typically have very high Q.

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#12
In reply to #10

Re: Coil Inductance Without and With Water in the Center

05/07/2014 9:01 PM

Water is very slightly diamagnetic and will tend to reduce the inductance by slight amount.

I got one of those strong Neodymium-Iron-Boron magnets and put it in a pan of water with the water level slightly above the pole of the magnet. I could see a slight dent in the water above the magnet by observing the reflection of a window screen in the water.

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#13
In reply to #12

Re: Coil Inductance Without and With Water in the Center

05/07/2014 9:14 PM

Very clever experiment! I've a stack of new Nd washer-style magnets and I'll check it out.

I know with very powerful fields (~10T) you can levitate a blob of water in mid-air (though toads are more cooperative).

Thanks!

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#18
In reply to #12

Re: Coil Inductance Without and With Water in the Center

05/08/2014 12:04 AM

If what you saw was a dent (depression) on the water surface, wouldn't that indicate that the water was slightly paramagnetic, rather than diamagnetic?

What kind of Pan? magnetic, para- or dia-?

Since you were viewing a reflection, I suspect it would be easy to confuse a depression with a hump.

This I will try myself... And I'll try it first in a glass pan...

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#20
In reply to #18

Re: Coil Inductance Without and With Water in the Center

05/08/2014 12:37 AM

Just tried it and took pix, but I noticed something else: the magnet acts *as if* it were hydrophobic in the extreme.

I put a stack of three Nd washer magnets into a plastic lid and slowly filled the lid with water with the goal of just barely covering the magnet. The water refused to do any such thing.

Poured in more water so that it is now roughly ***3 mm*** above the magnet periphery - and STILL the water won't cover the magnet, nor will it rise to much extent above the central hole.

Check this out. Have you ever seen a meniscus that high even with an oily object?

Offhand I'd say that water is diamagnetic.

.

Next I'm putting a small amount of surfactant in the water and see if a meniscus still forms to that height.

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#27
In reply to #20

Re: Coil Inductance Without and With Water in the Center

05/08/2014 12:32 PM

Great photo! That definitely is a major meniscus!

I didn't find any washer-type Nd magnets, so I can't really repeat your experiment.

This is a stack of disk-type Nd magnets, 2-1mm thick, 2-1.5mm thick, and one 5.1mm thick, with the thickest on top, all 10mm in diameter. My container is a molded glass salt cellar with an ID of 32.5mm and a fairly sharp inside top edge.

I did NOT observe any significant increase in meniscus depth over a non-magnetic object. Here you can see when the level of water was just starting to cover the top of the magnets, with a rather ordinary meniscus.

The next photo was taken when the water depth just barely covered the magnets by roughly 1mm:

I left part of a teapot in the background, to show that it was the source of the light that can be seen reflecting off the surface of the water. I could not discern any distortion of the flat surface at all.

I also tried bringing the stack of magnets up to a thin falling stream of water, and could NOT see any deflection. It was not a very good setup, so I did not bother taking any photos. I have many times observed the significant deflection of such a stream by an object carrying a static charge.

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#29
In reply to #27

Re: Coil Inductance Without and With Water in the Center

05/08/2014 12:51 PM

Last night after I took those photos I tried another experiment where I placed the magnets under that plastic lid which held just enough water to cover the bottom. The water's surface over the magnet formed a very shallow annular depression. Hard to see unless I moved back and forth whilst watching the change in the reflection. Took some photos but the effect was so subtle that it's not all clear from the photos.

Will repeat the experiment tonight with a fine grid in the background, reflecting off the water. I'm hoping this will make the depression and its shape much more obvious. I'll post the pix.

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#21
In reply to #18

Re: Coil Inductance Without and With Water in the Center

05/08/2014 1:03 AM

Hi dkwarner,

I am also seeing a slight depression in the water surface directly over the magnet.

In my previous post you can clearly see the water is 'pushed away' from pole, forming a deep meniscus around the magnet periphery. When the water depth is increased to the point where it finally covers the magnet, it is still pushed away, but not completely, resulting in a 'dimple' in the water's surface just above the magnet. This is what one might expect were the water diamagnetic.

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#22
In reply to #18

Re: Coil Inductance Without and With Water in the Center

05/08/2014 2:01 AM

1. Bulb reflection 'far' from magnet (but note distorted lampshade reflection near magnet).

.

2. Reflection of light bulb distorted by downward curvature of water surface near magnet periphery. Note especially the bulb reflection in the *convex* surface over the central hole where the field is weaker.
The depression in the water's surface has an annular shape, like a shallow, circular trough.

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#24
In reply to #22

Re: Coil Inductance Without and With Water in the Center

05/08/2014 4:44 AM

I liked the work, but an extra picture with no magnet, of the reflection, would have made it more obvious for us here.

Thanks in advance.

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#30
In reply to #24

Re: Coil Inductance Without and With Water in the Center

05/08/2014 12:55 PM

Repeating the experiment this evening with a fine grid in the background, reflecting off the water. This should make the shape and extent of the depression much more obvious.

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#31
In reply to #30

Re: Coil Inductance Without and With Water in the Center

05/08/2014 1:43 PM

I too liked Rixter's idea of the grid in the background. This time I used a hard drive assembly of 2 magnets, one N up and the other S up, attached to a base plate.The container is a glass coaster. This gives me a much larger surface; the ID of the coaster is about 85mm.

Here's the setup as seen from above, water just covering the magnets (I touched the uncovered areas with a spoon handle to help overcome the surface tension with a minimum layer of water covering the magnets):

...and here's the low angle view, with a sheet of paper taped to the outside of the window screen to remove confusing background:

If I get the angle just right, I can see some distortion of the reflection near the glass (the water surface was a couple of mm below the rim of the glass), but I don't see any distortion due to the magnet.

Just in case it makes a difference, our water supply is mostly snow melt, never having been underground, and therefore is very soft, with virtually no dissolved minerals.

If your water is hard, try repeating the experiment with distilled water.

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#38
In reply to #30

Re: Coil Inductance Without and With Water in the Center

05/08/2014 8:22 PM

By far the best way to see the effect is to observe a reflection of some detailed object. Move your head back and forth concentrating on some detail in the reflection. As this detail moves across the dent, you should see it distort.

Unfortunately, I don't know how to attach a video.

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#39
In reply to #38

Re: Coil Inductance Without and With Water in the Center

05/08/2014 8:29 PM

See my post below, #35.

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#40
In reply to #39

Re: Coil Inductance Without and With Water in the Center

05/08/2014 8:44 PM

It's definitely there, and you put to bed the possibility of surface tension. Good work.

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#42
In reply to #40

Re: Coil Inductance Without and With Water in the Center

05/08/2014 8:49 PM

Thanks.

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#25
In reply to #18

Re: Coil Inductance Without and With Water in the Center

05/08/2014 8:20 AM

It seems kind of counter-intuitive, but repelling the water creates a dent. Think of the opposite: if the water were attracted, it would "pile up" on top of the magnet.

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#26
In reply to #25

Re: Coil Inductance Without and With Water in the Center

05/08/2014 12:25 PM

Exactly.

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#28
In reply to #25

Re: Coil Inductance Without and With Water in the Center

05/08/2014 12:46 PM

I have indeed thought about it, but I can NOT come to a conclusion just yet...

Your concept of the magnet pulling water in and therefore, as you say, "pile up" on top of the magnet definitely has logic to it.

On the other hand, since magnetic forces diminish rapidly with distance, the force pulling down on the water directly above the pole should be much greater than the force pulling in on nearby water, thus creating a depression.

Was the magnet you used a disk, a washer, or some other shape?

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#32
In reply to #28

Re: Coil Inductance Without and With Water in the Center

05/08/2014 1:47 PM

Go on YouTube and search under 'ferrofluids' if you want to see really COOL effects with magnets.

This topic is so interesting in its own right that it really needs its own thread.

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#33
In reply to #32

Re: Coil Inductance Without and With Water in the Center

05/08/2014 1:56 PM

Yes, I've had that in the back of my mind while taking the recent photos, but now I've got to get some work done!

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#36
In reply to #28

Re: Coil Inductance Without and With Water in the Center

05/08/2014 4:52 PM

The magnet I used was a 3/4" diameter cylinder, 1/2" high. I bought a couple from ForceField.

http://www.forcefieldmagnets.com/catalog/

I did this some time ago and I didn't take any photographs.

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#46
In reply to #12

Re: Coil Inductance Without and With Water in the Center

05/09/2014 12:02 PM

Some other experiments...

http://www.wondermagnet.com/diamagh2o.html

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#23
In reply to #10

Re: Coil Inductance Without and With Water in the Center

05/08/2014 4:41 AM

Good Post.

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#14

Re: Coil Inductance Without and With Water in the Center

05/07/2014 10:46 PM

Use a capacitive sensor. These from Balluff are great, they have a way of discriminating out foam which can throw off most other sensor technologies. I have used them in some really ugly situations where I had to detect through non-metallic walls like tanks and pipes. Very reliable and repeatable.

http://www.balluff.com/balluff/MUS/en/products/smartlevel.jsp

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#15

Re: Coil Inductance Without and With Water in the Center

05/07/2014 10:50 PM

National Semiconductor has an inexpensive automotive chip for detecting low coolant. It contains a blocking oscillator circuit that trips when pipe has no anti freeze or water in it. I believe this is also used to detect low wind shield washer fluid.

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#16

Re: Coil Inductance Without and With Water in the Center

05/07/2014 10:59 PM

What will be the values of current in coil if there is air,stationary water,flowing water through the pipe?.

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#17

Re: Coil Inductance Without and With Water in the Center

05/07/2014 11:42 PM

Commercial toroidal conductivity measurement uses a sheathed coil (insulated from the medium) to measure conductivity of liquids:

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#37
In reply to #17

Re: Coil Inductance Without and With Water in the Center

05/08/2014 4:58 PM

Conductivity, not permittivity.

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#34

Re: Coil Inductance Without and With Water in the Center

05/08/2014 1:59 PM

I love this blog but I cannot see for certain if its the magnet causing this effect, or the type of metal and/or any finishing that has been done to it......

Surface tension is a science on its own.

I do not see anything that says that this effect is NOT simply surface tension reacting with the metal of the magnet in some manner, sorry.

You would need an identical magnet(s), degauss it (them) completely and make exactly the same test......my personal bet is that the effect will stay the same with no magnetism.....the reason being that the water has to be so "thin" to show the effect.....I would guess a wax disk may show similar effects.....

Sorry!

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#35
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Re: Coil Inductance Without and With Water in the Center

05/08/2014 3:55 PM

This goddam editor just trashed my post. Pix, everything.

Second attempt.

Repeated last night's experiment, this time with a grid in the background:

.

Next I removed the magnet from the lid and placed it beneath a thinwalled, shallow plastic 'tray' (the blisterpack from a Radio Shack protoboard) to mitigate any effect due to surface tension. As you can see, there is a slight depression in the water's surface directly over the magnet:

.

Next, some pix of the setup:

Sorry about the orientation, but I am writing this from my mobile. I rotated the image but it is still on its side, as before. We'll just have to make do. CR4's editor does not play well with Android's version of Chrome.

.

.

.

Note the laptop in the background. For the grid I launched M$ Paint, flood-filled the Canvas with black, turned on Gridlines and set the Zoom to max.

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#43
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Re: Coil Inductance Without and With Water in the Center

05/08/2014 9:10 PM

I really like your idea of using the laptop as a grid source. I use a Mac, so can't use the exact same method, but I can easily set up a grid in CAD.

I'm still not quite sure what I'm seeing. As the grid lines approach the left and right sides of the magnet in your first photo, they diverge. Then when they pass from the magnet to the hole, they converge. It's harder to see in the lines from front to back, but the same is true. There is definitely an annular distortion.

I'm gonna do some more experimenting myself. I'll be back tomorrow!

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#44
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Re: Coil Inductance Without and With Water in the Center

05/09/2014 5:37 AM

Suggestion: rotate your grid 45 degrees so that *both* sets of gridlines will more clearly reveal the shape of the distortion. Note that the front/back gridlines in my photos reveal little about the curvature where they're parallel to the line of sight. Rotating the grid should help.

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#45
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Re: Coil Inductance Without and With Water in the Center

05/09/2014 10:26 AM

I'll try it!

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#47
In reply to #44

Re: Coil Inductance Without and With Water in the Center

05/09/2014 2:34 PM

OK, I took about 50 more photos under various circumstances. Here was my best setup:

Yep! It is indeed directly on top of my laptop keyboard!. Obviously I was veeerrry careful of the water.

Here is the best result with the XY grid:

This clearly verifies the divergence of the grid as the lines approach the top of the magnet, just as in your photo.

Here's the one with the grid rotated 45°:

I definitely found it harder to get a good photo with the 45° grid, and the result is not as obvious.

I am definitely sure that there is a depression in the surface of the water over the magnet, when the depth is adjusted correctly. I'm not at all sure whether that indicates attraction or repulsion of the water by the magnet...

I do still have more experimenting to do!

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#48
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Re: Coil Inductance Without and With Water in the Center

05/09/2014 3:58 PM

Nice pix!

I reckon that if you raise your camera a bit higher where you use the 45-degree grid, you may see it gives better detail inside the depression. Moving the setup farther from the screen will also give you a finer grid spacing if need be.

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#49
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Re: Coil Inductance Without and With Water in the Center

05/09/2014 6:44 PM

Unfortunately, there is only a very narrow range of angle where I can see the reflection off the surface of the water. Even a slight raising of the camera, and the reflections disappear.

I could easily make a finer grid; this was a 0.1" spacing. With this camera, (a Cannon Powershot Elph 330HS), I can't focus close enough when I zoom in to see the small area from a greater distance. These pix are cropped from larger areas. I took your idea of cutting a blister pack to make the container, and set it on top of an inverted plastic storage container to raise it up to a level where I could get the camera in close. That container also protected the keyboard somewhat in case of a small spill.

If I find time to try a finer grid, I'll print it on a laser printer, and put a light behind the paper, thus avoiding any chance of spilling on my keyboard.

I do also plan another try on the dripping water...

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#51
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Re: Coil Inductance Without and With Water in the Center

05/10/2014 12:29 PM

Here are the results with a 0.05" grid printed on paper. All have been converted to B&W, which seems to improve the visibility of the lines. This time I used a much larger and deeper container, a glass storage container about 15 cm square, so I have a large flat surface of water. This way I was able to use some zoom on the camera. The small magnet is on top of the stack of 10mm magnets. Still, the HV lines seem to work much better than the 45° lines.

In this last photo, I've used less magnification to show a bit of the paper at the top of the image:

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#58
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Re: Coil Inductance Without and With Water in the Center

05/10/2014 8:08 PM

Those are great pics. Thank you.

.

A depression indicates repulsion. Attraction would create a lump as more is brought over, as is the case with gravity causing elevated mounts in the oceans above massive underwater mountains.

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#50
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Re: Coil Inductance Without and With Water in the Center

05/09/2014 6:55 PM

Good idea for the grid. No doubt that it is uniform.

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#57
In reply to #35

Re: Coil Inductance Without and With Water in the Center

05/10/2014 8:03 PM

These are great. Thank you.

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#41
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Re: Coil Inductance Without and With Water in the Center

05/08/2014 8:46 PM

Evidently that high 'meniscus' I reported from last night's experiment...

...has also been seen at Japan's High Field Magnet Laboratory. Here the water is driven completely away by the 8T field:

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#52
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Re: Coil Inductance Without and With Water in the Center

05/10/2014 3:36 PM

Now that is what I call a meniscus!

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#53
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Re: Coil Inductance Without and With Water in the Center

05/10/2014 3:48 PM

Lol. You might say that!

Was reading somewhere that they dubbed that the 'Moses Effect' in reference to the Biblical parting of the Red Sea.

I wonder, were those Israelites diamagnetic and Pharaoh's infantry (gasp) paramagnetic?

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#54
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Re: Coil Inductance Without and With Water in the Center

05/10/2014 4:05 PM

I would like to see what would happen if they placed a deep container full of water on top that magnet in the lower pic. Would the field carve out a 'bubble'? What would be its shape, etc?

Years ago I worked at a lab where we had several large superconducting magnets. One of them produced a max field strength of 14.1 Tesla and had a sample chamber large enough to accommodate a frog or a small water balloon. They do float, btw!

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#55
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Re: Coil Inductance Without and With Water in the Center

05/10/2014 7:00 PM

I've see pictures of that. I've always assumed the frogs were unhurt in the process. Is that true?

What alloy is used for the pole pieces of such a strong superconducting magnet?

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#56
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Re: Coil Inductance Without and With Water in the Center

05/10/2014 7:32 PM

It doesn't hurt them AFAIK. Mr. Frog seemed happy as a clam when we put him back in the woods where we found him.

Pole pieces? I'm not sure what they used but I strongly suspect these were coreless solenoids, as anything in that strong a magnetic field, and in such a small volume, as well, would immediately saturate and stop being a pole-piece. In the photo above, I reckon that's a protective aluminum end-cap on their magnet.

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#61
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Re: Coil Inductance Without and With Water in the Center

05/12/2014 9:52 AM

I understand it's superconducting. Iron definitely would be saturated. It's the closest thing to antigravity. It's hard to get much better than supporting a body by the electron shells around the atoms!

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#62
In reply to #55

Re: Coil Inductance Without and With Water in the Center

05/12/2014 6:00 PM

Holmium is sometimes used to form pole pieces in very powerful magnets with significant benefit.

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#63
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Re: Coil Inductance Without and With Water in the Center

05/12/2014 11:36 PM

Thanks for the very interesting link! I had probably heard/read about holmium before, but had totally forgotten about it...

I have to work this week, so may not get back here 'till next week!

Dick

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#64
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Re: Coil Inductance Without and With Water in the Center

05/15/2014 4:45 PM

I wonder what it would be like to put your finger in there and feel the magnetic field.

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#65
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Re: Coil Inductance Without and With Water in the Center

05/16/2014 6:25 PM

The force on a diamagnetic object is zero in a constant magnetic field no matter how strong, this is because the magnet has both a north and south poles with opposite forces. However these superconducting magnets do not produce a constant field. The field is a gradient - it changes with distance. Thus the field exerts a different force on the north and south pole of the diamagnet since they are at different places in the gradient. So it is the product of the field and the spacial rate of change of the field that is the important parameter.To levitate water one needs a field times the change in field divided by distance greater than 14 T^2/cm. There is a nice paper at http://www.sciencedirect.com/science/article/pii/S0273117709005985 that discusses this for various materials. This means that the magnetic field has to be very high at a distance that is a long way from the center. I know of no magnet with a bore diameter of more than 2 inches that meets this requirement. In high field research magnets you can reach that value for small diameters but, in general, as you approached the high field region from the 'top,' your hand would feel normal 1 g acceleration of gravity and it would gradually reduce to feeling weightless at 0 g (where the gradient is highest). As you got to the center of the magnet (where the gradient is zero) you would feel 1 g normal gravity again. As you moved below center your hand would feel increasing weight until it reached 2 g (gravity + repulsion, except now downward) and then gradually reduce to 1 g as you leave the field.

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#59

Re: Coil Inductance Without and With Water in the Center

05/11/2014 4:44 AM

Yuri B,

What frequency range are you considering for your coil?

What is the pipe material?

How thick is the pipe material?

What ratio of water / ethylene glycol?

Have we answered your question to your satisfaction?

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