Eric, about signal strength

[wyndham]

I bought some Lentz wire awlile back, thinking about making a coil. Last night Iwrapped a glass tube(1" dia) with about 40 turns and applies a 6 v DC current to this( a wall plug in from a Sony Walkman). The idea was to create a magnetic field, which I did.
But, I tied a small super magnet to a string and lowered it into the field, it reacted, mildly. a slight pull to one side or the other as it was in the field.
I they tried black sand, pouring it down the side of the glass. I hoped to observe either a slowing or capture but neither occurred.
If the coil is similar to the TX coil in that it creates a field and induces an eddy current, is the particle size so small as not to produce an eddy current and a magnetic flux that would be noticed in the field I generated?
Is the voltage or amps too low or time too short or that there is a min size to a particle that can have an effect.
I figure If I can make a visible test like this I may have a better understanding of the process. thanks PS anybody else want to weigh in, please, I'm in over my head. Wyndham

 

[Kev]

Hi Wyndham,
I'll ave a go since nobody else wants to take it on. I'm probably way off beam anyway. Eric will likely sit back for a bit and let others comment , he likes to get us thinking, I think He'll come in and put us on the right track soon.

A 1 inch coil without an iron core to entrain and focus the field is not going to have the same effect as a magnet, although that is what you've made. 40 turns may also present a relatively low resistance to your power source, and it may have gone into current limiting mode.

Your coil would have an inductance of ruffly 40uH, whereas a detector coil is in the region of 300uH, even then it is very hard to see any noticeable effect on objects. You will often see a "magnetic shadow" on wet beach sand as the coil is swept across it, but then the current in a detector coil is alternating, and this is the key to metal detection, the eddy currents in a target build in opposition to the field generated around a coil, but only when the current in the coil is changing, either up or down. (Lenz's law)

Any observable effects without instruments will solely be according to electrodynamic forces which will not be wholly representative of what's really happening in a detector.

Good on you Wyndham for experimenting, and sharing your observations with us.
Maybe Eric has an experiment that demonstrates these effects?

Cheers
Kev.

 

[Reg]

Hi Wyndham,

Kev is most likely correct on the current limiting of your test power supply. Most of the small "Wallwarts", the common wall plug power supplies have a relatively high internal impedance and will self limit any current. So, most will not provide enough current to do the job. However, if you look around, you can find some that are capable of a few amps.

Kev is probably very close to the actual inductance of your coil also. Here is a link that will help you be a little more accurate in determining the inductance.

http://my.athenet.net/~multiplx/cgi-bin/airind.main.cgi.

You can select either millimeters or inches which makes this simple program nice. Also, once you fill in the blanks, the answer will come up as approximate or .1% accurate. Select the .1% to be the closest, providing your measurements are correct. This program works very well for determining the number of windings needed when building larger coils such as those for a PI also.

One thing about your tests is it will show you how much of an obvious effect occurs when a magnetic field is applied in a controlled environment. With a little adjustment of your initial design you can see what happens when a coil of a similar inductance as those of the PI's, i.e. 300uh, has on something such as black sand.

Now, you will need a little better supply that will allow you to control the current to better see what can happen. Also, you will probably need heavier wire also. I suspect you will need quite a bit of current to be able to see anything obvious.

Like Kev said, "good on you" for experimenting and thinking of this type of experiment.

Reg

 

[wyndham]

Ok, this is what I needed, another set of eyes on the project.
The Idea that the current needs to alternate is something that stands out.So it is the rate of change and the alternating signal that must be taken into accout.
My first thought was that like a cyclone, the greater the pressure change increses the rotation, so would the increase in either voltage or winding increase the effect on the sand, but from what you are saying the alternating pulse induces the magnetic flux(when voltage and windings are also increased to the approaite size). This is correct, roughly?
The fine lentz wire was used because of the use in coils, but I do have others and will try several.
Thanks again, this gives me a toe hold on a rather difficult(for me) concept.
Wyndham

 

[wyndham]

The site says that the inductance is based on a single layer. Are the values about the same or quite different if the coil is overlapping itself an the width is about 3/4 inch so there is no single layer in my setup.
Wyndham

 

[Reg]

Hi Windham,

I have used the calculations at this site to determine my PI coils which are not single layer and the calculated inductance is very close to the measured inductance. It is so close, that I only measure the inductance if I think I have made a mistake on the number of turns involved.

In other words, I have found the calculator to be accurate even on coils having multiple layers such as a PI coil.

In your case, my guess is the calculated inductance would be well within 5% of measured, but more likely within 1%.

Reg

 

[Frank-S]

wyndham,
Try put-ing,a 3/4" bolt in your 1" glass tube with 40 turns of wire.place a
sheet of paper over it,use metal shaveing over it in place of black sand.see what it
will do.also try black sand.Also try your super magnet at one end of bolt,turn current
off and on.