DIY specimen holder

ChrisR · Post by **ChrisR** » Wed Feb 05, 2014 9:36 am

There are also spherical magnets. (Found up to 2", search for "sphere" not "ball" for larger ones).
If the croc clip is suitably magnetic, and has an adequate socket, then it would just slide around with no further parts.

Not for rhinocerous beetles, perhaps.

genera · Post by **genera** » Wed Feb 05, 2014 9:40 am

You can use a second identical magnet . . .

The description of the product elf provided a link to makes the point that the countersunk surfaces of the two magnets will need to be of opposite polarities.

Edward Ruden · Post by **Edward Ruden** » Wed Feb 05, 2014 7:28 pm

ChrisR wrote:There are also spherical magnets.

One complication with this is that a steel pin (with or without an attached magnet) seated to a spherical permanent magnet will be attracted to the fixed magnetic pole of the sphere. This will limit stable seating to places near the pole only, where a significant downward component of force will allow friction to hold it in place. A steel ball magnetized by a magnetic base does not have this problem because the magnetic field lines freely flow from the base to the pin, creating an attractive force toward the center of the sphere (or thereabouts) at all locations.

ChrisR · Post by **ChrisR** » Sat Feb 08, 2014 5:31 pm

will be attracted to the fixed magnetic pole of the sphere

I passed a charity shop which had an exectutve toy on show. Small black magnetic balls. Playing with just two, they showed no such anisotropy. In other words they stuck together wherever they were put together.

(It occurred to me that a mass of these balls would be quite useful for propping up randomly shaped small items for photography. The balls stay wherever they're squished to.)

Edward Ruden · Post by **Edward Ruden** » Sat Feb 08, 2014 7:37 pm

ChrisR wrote:I passed a charity shop which had an exectutve toy on show. Small black magnetic balls. Playing with just two, they showed no such anisotropy. In other words they stuck together wherever they were put together

That's not quite the same thing as a countersunk magnetic disk on a magnetized sphere. A second sphere is free to rotate so as to be antiparallel to the first sphere's polarization. If you hold the two spheres between thumb and forefinger so as to prevent them from rotating, do they still stick to each other regardless of relative position?

The ball bearing on a magnetic base with a countersunk steel pin is on my to-do list. The list is quite long, though, so it'll be a while before I try.

ChrisR · Post by **ChrisR** » Sat Feb 08, 2014 8:43 pm

If you hold the two spheres between thumb and forefinger so as to prevent them from rotating, do they still stick to each other regardless of relative position?

Yes.

Edward Ruden · Post by **Edward Ruden** » Sat Feb 08, 2014 9:20 pm

ChrisR wrote:
If you hold the two spheres between thumb and forefinger so as to prevent them from rotating, do they still stick to each other regardless of relative position?
Yes.

That's interesting. Those magnetic spheres are not as simple as they appear. I can think of only two possible explanations. The first is that each ball contains a magnetic monopole: one north, the other south. A quick check of the last few years' Nobel Prizes in Physics rules that out; the inventor would have won. The other is that holding the balls does not prevent an internal magnet from rotating. There may be a smaller spherical magnet inside a cavity filled with oil, say. You can test the latter hypothesis by sticking one ball atop the other (thereby aligning and locking their internal magnets in place), and approaching the pair from below with a weaker magnet, pole up. If it is attracted to the balls, turning it upside down so the other pole is up should cause it to repel. However, if you separate the two balls and repeat, the small magnet should only be attracted.

ChrisR · Post by **ChrisR** » Sun Feb 09, 2014 2:55 am

There are of course less spurious possibilities, but you seem determined

.

boomblurt · Post by **boomblurt** » Sat Feb 22, 2014 11:31 pm

Here's an idea for a swiveling/rotating stage:

Ingredients: one ball bearing, two small neodymium magnets, galvanised steel squares.

It is super cheap and easily modified (eg put some cork board on top to pin specimen).

AndrewC · Post by **AndrewC** » Sun Feb 23, 2014 9:16 am

So what would be really nice is a magnetic metal spherical cap with height significantly less than the sphere radius, and maybe even truncated so the radius of the flat plane defining the cap is less than the sphere radius (imagine just the end off an apple core ?) being held on an annular magnet. Subject is then held on a pin above the centre.

This gives you a nice goniometric stage free to move in any axis.

So far this has defeated me. I did try making a mould and filling it with resin loaded with iron filings but there wasn't enough iron content.

Also tried with a plastic sphere with a metal ball bearing rolling on the inside to couple to the supporting magnet but that also wasn't strong enough.

I've been watching out for some sort of steel sphere I could try and cut down to size and then fill with resin or plasticine (though the potential for self harm when trying to cut a metal sphere on a power saw is high) but haven't found anything yet. The original sphere needs to about 10cm in diameter.

I did make a "gravity mount" using this concept with a section out of a plastic sphere but it was just not heavy = strong enough.

I'm sure someone will probably make a nice sketch showing what I mean but I leave that as an exercise for the reader

ChrisR · Post by **ChrisR** » Sun Feb 23, 2014 11:55 am

Start with something like THIS ?

AndrewC · Post by **AndrewC** » Mon Feb 24, 2014 2:13 pm

Are you trying to turn me to a life of crime ?
I did think of pilfering a mixing bowl but it is stainless steel which isn't one of the better known magnetic materials

johan · Post by **johan** » Mon Feb 24, 2014 3:16 pm

http://www.kellydavies.co.uk/products/h ... -iron-bowl

abpho · Post by **abpho** » Mon Feb 24, 2014 3:38 pm

Ingenious idea. Well done.

ChrisR · Post by **ChrisR** » Mon Feb 24, 2014 4:53 pm

did think of pilfering a mixing bowl but it is stainless steel which isn't one of the better known magnetic materials

It might be ferromagnetic, if it's ferritic stanless steel.
Not if it's austenitic though.

--

I asked a magnet supplier to play with his spheres. He reports that a 19mm magnetic sphere and two steel discs works well.