# [Solved] What affects magnetic attraction?

This seems like a simple issue but Google has not helped.

I want to use a permament magnet (moved into, or out of place by a servo) to decouple model trains. I am using a piece of paper clip soldered to a piece of guitar string such that the force of the magnet on the paper clip causes the guitar string to deflect. This image should convey the idea.

What I have not been able to figure out is what changes to the paper clip affect the distance at which the magnet will cause the spring to deflect.

Things I have been trying to experiment with include

• the mass of the clip (a longer piece)
• the area of the clip facing the magnet

but my experiments have not been conclusive

Everything I have so far found with Google is too theoretical. I am just looking for a simple rule of thumb equivalent to “a bigger hammer hurts more”

Has anyone any knowledge of this, or any useful links

…R

Well, I see an incomplete magnetic circuit there (or indirect). If you can route the other pole of the magnet around so the field lines from both poles are near the clip, the attraction will be much stronger. So, longer piece of clip (the "face" you mentioned...) and turn the magnet so both poles are adjacent to it.

Things like guitar string diameter, length and tension will also have a big impact. Why bother with the magnet, vs just having the servo move the wire directly?

CrossRoads: Things like guitar string diameter, length and tension will also have a big impact. Why bother with the magnet, vs just having the servo move the wire directly?

Yes I understand how all those other variables are relevant. For various reasons that seem sensible to me (such as the space available) please consider them all to be fixed.

The spring and paper clip are on a railway wagon. The magnet and the servo will be under the baseboard.

aarg: Well, I see an incomplete magnetic circuit there (or indirect).

You are clearly a few pages ahead of me in the text book :)

If you can route the other pole of the magnet around so the field lines from both poles are near the clip, the attraction will be much stronger. So, longer piece of clip (the "face" you mentioned...) and turn the magnet so both poles are adjacent to it.

I will investigate that. However it may not be easy because magnets like to attach themselves by their poles rather than their sides. The magnets I have are about 4mm diameter and 2mm high.

I found that if I have nothing "stuck" to either pole and present one of the poles to the paper clip there was very little force, whereas if I "stick" one end of the magnet to a piece of steel set screw the other pole has a strong force. I am guessing that without the piece of steel the magnetic lines wrap around and cancel out - like a loudspeaker without a baffle.

In any case, if I get the magnet in the best orientation there will still be a possibility of "improving" the paper clip and advice about that will be very welcome.

What is it about the size or shape of the piece of "iron" that affects the force a magnet imposes on it?

Thanks all.

...R

whereas if I “stick” one end of the magnet to a piece of steel set screw the other pole has a strong force

Magnets are dipoles and the magnetic field strength at a point in space depends on the distance between the poles (i.e. the North and South faces). Your magnets are physically small so the measurable field doesn’t extend very far.

The set screw is acting to increase the effective distance between the faces. You could use other shapes of magnetic materials to create a strong field where you want it (the magnetic circuit mentioned above). Think of the horseshoe magnet.

What is it about the size or shape of the piece of “iron” that affects the force a magnet imposes on it?

Both size and shape would affect the force. Smooth surface is better. Same shape (footprint) as magnet is better yet. A “magnetized” piece of iron is even better. Replacing the piece of iron with another magnet (oriented correctly) yields more force. The type of magnetic material and its size/shape also play a role.

For various reasons that seem sensible to me (such as the space available) please consider them all to be fixed.

I guess this rules out magnetic sensors and reed switches?

jremington: The set screw is acting to increase the effective distance between the faces.

That's what I think I was thinking without being able to express it clearly like you have.

dlloyd: Both size and shape would affect the force. Smooth surface is better. Same shape (footprint) as magnet is better yet.

This may be interesting. I think you are you saying that a magnet will exert more force on a piece of iron that has a matching footprint compared with a piece of iron with the same cross-section but a different shape. Maybe the head of a tack would be better than piece of paper clip. More experiments.

By the way it will be essential that the paper clip (or whatever) never actually touches the magnet.

I guess this rules out magnetic sensors and reed switches?

I'm not sure what you have in mind but I am not trying to detect anything or switch anything - I just want to cause a small amount of movement in the spring wire.

...R

If you have space, use an L-shaped piece of soft iron or other magnetic material to (effectively) bring the opposite pole of the magnet close to the wire as well. Again, think horseshoe magnet.

dlloyd:
Same shape (footprint) as magnet is better yet

This sparked off a completely new and very productive train of thought.

Using a setscrew provides the necessary footprint and allows adjustment of the maximum movement - which I was also having trouble with.

I started with an M3 setscrew with the head cut off which worked, but was much too obtrusive. Then I tried a 10BA (I think - the label is gone) setscrew with the CSK head still attached and it works very much better than the paper clip.

Karma for you and I will mark this solved.

…R

The magnet and the servo will be under the baseboard.

Why not replace the magnet and servo with a coil? No need then for moving parts under the baseboard (always difficult to maintain). Just wind a coil onto a piece of steel (set screw?) in an electric drill.

Henry_Best: Why not replace the magnet and servo with a coil? No need then for moving parts under the baseboard (always difficult to maintain). Just wind a coil onto a piece of steel (set screw?) in an electric drill.

Magnetic uncouplers work with transverse magnetic orientation (right angles to the track). The permanent magnet and electromagnet products all do this.

I can't help think that making use of diamagnetic's would yeild very good results. Try one of these to start with diamagnic kit

Mark

A little permanent magnet attached to a servo arm seems a very simple solution to me. I have a lot of the magnets from a puzzle/game I bought years ago. A servo is very simple to control with an Arduino, compared with an electromagnet.

In the past I have not had success with winding electro magnets so I want to steer clear of that.

AFAIK the Kadee couplers use a long bar magnet or a very powerful (and expensive) electromagnet oriented along the track so it affects the pair of couplers joining two wagons at the same time. I only need to affect one of the couplers. What I am trying to make is a variant of the Alex Jackson coupling.

From a quick look at Wikipedia diamagnetism seems to be about repulsion. I need attraction (badly :) )

...R

Robin2: AFAIK the Kadee couplers use a long bar magnet or a very powerful (and expensive) electromagnet oriented along the track

No, the Kadee system is oriented across the track.

Sorry, I meant the physical orientation. I take it from what you say that the poles are on the long sides of the bar magnets - I had not realized that.

I think that is to cause a sideways uncoupling action. I need a vertical downwards action.

Thanks.

...R