# Determining the North and South Magnet



## Grandcheapskate (Jan 5, 2006)

Hi Guys,
As everyone knows, in order to get an armature to turn you need a north and a south magnet. While it is easy to determine the north and south ends of a bar magnet (using a compass is the easiest method), it is not so easy to understand which magnet on a pancake chassis is considered north and which is considered south. This is because the poles on these types of magnets are on the inside (the curved side toward the armature) and outside edge of the magnet. So is a north magnet north on the inside or north on the outside?

When using a compass to determine the north and south pole of a magnet, you can take a magnet and move it close to the compass. If the north end of the needle points toward the magnet, you are looking at the south side of a magnet, and vice-versa. Remember, opposites attract.

In testing magnets in a T-Jet, the front magnet is south on the inside curved section and north on the outside edge. The rear magnet is just the opposite.

In discussing this the other day, we believe the front magnet is considered the north magnet and the rear magnet is considered the south magnet. We believe since the magnetic field runs through the magnet from south to north the direction of travel is north and hence the magnet is considered north.

Does anyone have a degree in magnetism to shed light as to whether my explanation is correct?

Thanks...Joe


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## Rich Dumas (Sep 3, 2008)

If you are checking Johnny Lightning/Auto World magnets the blue one goes in the front and the white one goes in the back. If you place a compass facing the armature side of a blue magnet the compass arrow will point towards it and if you put the probe of a magnetometer on that face it will read a positive value. With the white magnet the compass arrow will point away and the magnetometer will read a negative value.
If you have loose magnets with a color coding that you are not familiar with you would want to start by pairing them up, if two magnets attract each other they must be opposite polarities. If you have a magnet that you know the polarity of you can use it to determine the polarity of an unknown magnet if you do not have a compass or magnetometer handy.
T-Jet type magnets vary greatly in strength. I have measured values as low as 300 gauss and as high as 1100 gauss. It is best to use pairs of magnets that are of equal strength. A fanatic tuner would only use magnets that are within a few gauss of each other, for more casual applications those within 100 gauss would probably be OK.
The strongest regular T-Jet magnets that I have measured are the green/orange ones, those are about 650 gauss by my measurements and the green magnet is North.


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## Grandcheapskate (Jan 5, 2006)

Hi Rich,
I guess you misinterpreted my question.

Given two T-Jet style magnets, which one is considered the north magnet? The magnet which is north polarity on the inside (armature side) or north polarity on the outside edge? I know which magnet goes in the front and rear chassis pockets, but I am wondering about how the magnets are classified as north and south. I believe the one in the front pocket is the north magnet, but why? Since all magnets have a north and south pole, why is one considered north and the other south?


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## Rich Dumas (Sep 3, 2008)

I get your point, I have never seen a discussion of that subject. At the end of the day the car either runs in the right direction or it doesn't. Sometimes issues of polarity depend on your point of view, for example I am a retired chemist and chemists believe that electrons flow from where there is a surplus of electrons (positive) to where there is a deficiency of electrons (negative). On the other hand an electrical engineer would say the opposite, that the electrons flow up from ground. Who is right, in my opinion everybody! If I am speaking to a chemist I use the first convention, if I am speaking to an engineer I use the other, it is important to speak to people in a language that they understand.
In the hopes of finding a better answer I consulted the college physics book that I keep close at hand, the chapter on magnetism was mostly concerned with electrostatic charges however.
I tried a Wikipedia search and found this:
"A very common source of magnetic field found in nature is a dipole, with a "South pole" and a "North pole", terms dating back to the use of magnets as compasses, interacting with the Earth's magnetic field to indicate North and South on the globe. Since opposite ends of magnets are attracted, the north pole of a magnet is attracted to the south pole of another magnet. The Earth's North Magnetic Pole (currently in the Arctic Ocean, north of Canada) is physically a south pole, as it attracts the north pole of a compass. A magnetic field contains energy, and physical systems move toward configurations with lower energy. When diamagnetic material is placed in a magnetic field, a magnetic dipole tends to align itself in opposed polarity to that field, thereby lowering the net field strength. When ferromagnetic material is placed within a magnetic field, the magnetic dipoles align to the applied field, thus expanding the domain walls of the magnetic domains."
From that I would gather that if the arrow of a compass is facing a magnet that the magnet has its South pole on that side. Of course there is a North pole on the other side of the magnet.
I also looked at the article on DC motors, but that made no mention of magnet polarity.


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## Milton Fox Racing (May 27, 2014)

Would it help you @grandcheapskat to think of your question in the terms of the magnetic field they create instead?

Not to confuse the issue further, but since the dont actually touch - electronically (as above) the individual magnets have a different magnetic field as a horse shoe shape. Think of them as two bar magnets that are curved.











When in place though the two halfs of the slot car magnets *together with the armature in contact with both magnets* essentially form a closed ring field instead. So when inside the slot car the forward moving out side edge is the North magnet and the forward moving inside edge becomes the South magnet. Remember the forward movement of the slot car is created by the drive train (gearing) of the slot car not the magnets themselves. If you reversed the wiring on your track the car would not run in reverse it will try to turn around and still run forward to the shape orientation of the car.


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## alpink (Aug 22, 2010)

all these post are interesting and correct to a great degree.
my understanding of magnetism I cannot convey in any better terms.

I think the bottom line is to know the correct polarity of the magnets in the chassis.
then use a compass to determine the INSIDE pole of each magnet.
using that as a standard will help sort out magnets a little easier.

when I was 1/24 drag racing and I got into the high end ($400.00+) funny cars I had a motor builder explain some facts a bout magnets and how segmenting them within the standard semi-circle can create more high end power.
it is very interesting and exciting, though I understand the principles involved I do not remember correct terminology and therefore cannot easily convey them.

as Rich has pointed out, even in elctro-magnetic sciences and the very basics of electricity there are conflicting ideas about which direction electrons flow.

so, although I agree with theorems, vocabulary to explain can be different for folks and maybe not make sense at times.

it would be interesting if someone could put these ideas in simple laymans' terms without overstating principles.


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## Rich Dumas (Sep 3, 2008)

If you have a T-Jet magnet that you know goes in the front position you can use that to determine the polarity of unknown magnets and therefore you won't need a compass or a magnetometer. If you position the magnets so the insides face each other and they attract then the unknown magnet must go in the back position, if they repel it must go in the front position.
If you are going by the armature facing side of a front magnet you would have to call it a South magnet. If you were going by the outer side of that magnet you would have to call it a North magnet. Maybe we should have a poll to determine what to call which magnet. Actually North works better for me.


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## Grandcheapskate (Jan 5, 2006)

Rich Dumas said:


> If you have a T-Jet magnet that you know goes in the front position you can use that to determine the polarity of unknown magnets and therefore you won't need a compass or a magnetometer. If you position the magnets so the insides face each other and they attract then the unknown magnet must go in the back position, if they repel it must go in the front position.
> If you are going by the armature facing side of a front magnet you would have to call it a South magnet. If you were going by the outer side of that magnet you would have to call it a North magnet. Maybe we should have a poll to determine what to call which magnet. Actually North works better for me.


 I look at it this way. When installed properly (as stated above), the front magnet is south on the inside (cupped) edge and north on the outer edge. The north side of the rear magnet is on the inside and the south pole is on the outside.

If you draw a line through the magnets in the direction of travel starting from the rear, it would go from south to north to south to north. I would therefore consider the front magnet as north since that is the leading magnetic edge in the convention we use in our slot cars.

I had magnets made and when they came, they were separated into batches of north and south. So I was wondering why they considered one as north and one as south. I guess it really doesn't matter. However they considered the poles, we can consider them any way we like.

So let's just call them front and back.

It would be like taking two straight bar magnets and asking which is north and which is south. If you installed one so the north pole was up and the other with the south pole up, you might consider the first as north and other as south.

Thanks...Joe


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