Magnetic Poles That Are Alike

Magnetic Poles are a Myth

Let's answer iii questions:

When you pause a magnet in half, why don't you lot get a north one-half and a southward half? How do the broken pieces somehow know where to have north and south poles?
Where is the line separating the north and south poles of a magnet, exactly?
If the magnetization direction of a magnet isn't perfectly forth the centrality, how far off is it?

The Broken Magnet Problem

A D4X0-ND cylinder magnet, highlighted to indicate polarity.

If a bar magnet is cut in one-half, it is not the case that ane one-half has the north pole and the other half has the south pole. Instead, each slice has its ain north and s poles. --Wikipedia: Magnetic Monopole.

Consider a simple cylinder magnet. The D4X0-ND is one/4" diameter 10 ane" tall. The n pole has a dimple on it. Nosotros say the magnet has a north pole on the dimple stop, and the south pole on the reverse end. What does this really mean?

Labeling the poles is a handy way of thinking nigh a magnet. Information technology describes how nosotros expect it to comport. If nosotros hang the magnet from a string or float it in water, information technology acts similar a compass. The north pole points in the general direction of Canada from here in the The states.

It also describes how it will behave with other magnets. A north pole will attract to a south pole. Like poles placed near each other will repel.

This polarity is often represented past coloring each half of the magnet a different color to denote the n and south poles. We even sell a few magnets colored similar this, such every bit the D84PC-RB and D68PC-RB.

Here's the supposed mystery: if we break a alpine cylinder magnet in one-half, why do each of the halves then have a northward and a south pole?

Poles are a Construct

Poles describe how a magnet behaves, non what it is.

All this business virtually the poles is a construct. It's made up. It's a helpful crutch to remember about how magnets behave. It is not, however, a good description of what the magnet actually is.

If nosotros look within a magnet, zooming in with a microscope, nosotros find no physical difference betwixt the little $.25 of magnet at the n pole compared to the piddling $.25 of magnet at the s pole. It all looks the same.

There are no physical differences between the top and lesser halves of the magnet.

What Magnets are: Magnetic Domains

A D4X0-ND cylinder magnet is fabricated up of many tiny magnetic grains, sintered together.

If you zoom manner in on a sintered neodymium magnet, you detect that the material is fabricated up of a bunch of very tiny magnetic grains or domains. Each grain is similar a tiny magnet. The whole magnet is made of many grains of magnetic material squished together. Each grain is magnetized in a single management.

If nosotros slice ane of those D4X0-ND cylinder magnets down the heart and map it with an electron microscope, the design of grains with their magnetization management all point in roughly the same direction, parallel to the axis of the cylinder. (That's why we call information technology axially magnetized.)

Zooming dorsum out, nosotros can measure magnetic field direction around the magnet, shown with a compass in the epitome. Instead of maxim the magnet has a north and south pole, we might more accurately say that all the little bits of magnet fabric add upwardly to single magnetization management, pointing upward in this depiction.

Expect a second – How do pole identifiers work?

An EPID2 Electronic Pole Identifier shows the "poles" by sensing field direction.

A magnetometer senses the same field, showing the direction as a positive or negative number.

According to this pole-less way of thinking about magnets, we see that the field direction both in a higher place and beneath the magnet faces up. Why then does the EPID2 Pole Identifier easily detect the north and south pole?

It'southward all nearly the field direction and how you concur the measuring equipment.

When we concord the EPID2 to a higher place the so-chosen north pole, the field direction is pointing into the device.
When we hold the EPID2 to a higher place the south pole, the field direction is pointing away from the device.

The same is true with other measuring devices. A magnetometer shows the field strength as positive with the probe held above the north pole, and negative above the south. Of class, simply rotating the measurement probe effectually 180 degrees higher up the same pole will produce the reverse result. Measured direction depends non just on where you identify the probe, but on which fashion it faces.

Two Out of Three So Far

Both halves of a cleaved D4X0-ND cylinder magnet show a north and a south pole at either end with the EPID2.

With this modified way of thinking virtually poles and magnetization management, questions 1 and ii seem to respond themselves.

How does each one-half of a broken magnet each get a north and a south pole? Information technology's easy, because each one-half is just like the original – an not-varying length of magnetic material all magnetized in one item direction. At that place's no physical pole-stuff in them, earlier or after y'all intermission it.

Where is the line between the north half and the south half? There isn't one. That coloring scheme is just a mode to depict the magnetization management, not an indication of whatsoever physical boundary inside the magnet.

What is the tolerance of a magnet's magnetization direction?

The angle betwixt a magnet'due south geometric centrality and magnetization management

We tin draw an arrow showing which mode the magnetization direction points. How perfect is this arrow? Is the overall management of magnetization perfectly aligned with the cylinder'southward axis? Or tin can the magnetization direction differ by some small angle?

Yes, this angle has a tolerance, a mostly accepted range for the angle between the cylinder's axis and the magnetization direction. For sintered neodymium magnets, this number is typically within a few degrees. Let's say less than 5°.

Why is this so? Once again, recollect that a magnet is a squished together (sintered) drove of tiny magnetic particles (grains). The magnetization direction of all the individual grains are not all perfectly aligned. Each grain is off-axis by some amount. The total magnetization of the magnet is the sum of all those piffling bits of magnetization. Since some of those tiny arrows are pointing slightly left or correct, the off-axis magnetization components tend to cancel each other out. When this cancellation doesn't quite sum upward to zero, you lot get a combined magnetization that'southward slightly off-centrality.

Incidentally, this misalignment of the grains is one of several reasons why neodymium magnets can't exist fabricated with a theoretically perfect N64 rating. Some of the strength is lost to grains slightly canceling each other out.

How practise you lot measure the magnetization axis?

Measuring privileged angle with a fluxmeter and Helmholtz coils.

Nosotros use a fluxmeter and Helmholtz coils to measure the overall strength of a magnet. When rotating a magnet around in the center of the coil, the fluxmeter reads the current induced in the coils. Some fancy electronics turn this into a value for magnetic flux. A little math converts that data to a number expressing the magnetic moment, expressed in weber cm. It's a great quality control check.

Those coils are arranged to sense a magnetic field in the vertical direction only, through the axis of the coils. When nosotros place a magnet in the coils with the north pole facing upwardly, I see a positive flux on the meter. With the s pole facing upward, I see a negative flux on the meter.

If I stick a theoretically perfect magnet on its side, I should come across nada on the meter, since in that location's no overall magnetization in that side-to-side management, up-downward in the coils. In practice, if the magnet's magnetization direction is off by a degree or two, at that place is a small amount of flux seen past the curl.

By measuring the flux on the primary (pole) axis and two others, nosotros can compute the bending of that magnetization vector. This is sometimes called the privileged direction.

To put some numbers to this discussion, we measured a single BX0X04 block magnet. It worked out to just a chip over 3°.

Allow'due south see you measure information technology without that fancy fluxmeter!

Measuring privileged angle with a DIY sensor and Arduino.

For these articles, we like to experiment with the simplest electronics we can get our hands on. It's a way of making the thing we're studying a bit more easily understood and accessible. We don't think all our readers should get out and build the device that follows. It's only an interesting investigation to inform your thinking.

There isn't a universally agreed upon definition for how to measure out the magnetization angle. How you measure it might depend more about your application and how yous're using the magnet. Let's consider two possible definitions:

Similar the definition of the world'south north pole location, the north pole point might exist where around the magnet its field direction points straight downwardly (through the center of the magnet). Setup a sensor at some stock-still distance from the center of the magnet. Rotate the magnet until it shows the direction points direct towards the eye of the magnet.
For a broader definition, we might consider the theoretically perfect field at a whole bunch of points around a magnet. Measure the actual field direction at each of those points, and figure the overall management is the average of the errors at each point.

For you geo nerds out there, definition #one is like World's North Magnetic Pole. Definition #2 is like Earth's Geomagnetic pole.

We pulled out our trivial Arduino and magnetic sensor used in our contempo article nigh Shipping Magnets. With the assist of a 3D-printed test rig, we measured the field at 12 points around a circle (with a radius of half-dozen inches) around a BX0X04 block magnet.

Overall, the measurements averaged to an error of only over ii degrees, in the aforementioned direction we originally measured with the fluxmeter. It's not a perfect match, only within 1° is pretty expert for our rough experiment!

P.South.: This whole story unremarkably doesn't affair.

In the vast bulk of applications, the small-scale angle of magnetization direction error doesn't matter. Sticking a magnet to a fridge? It however sticks strongest on the pole faces. Small changes in magnetization angle don't seem to affect pull forcefulness in our tests.

Designing a magnetic sensor to switch off a laptop when it closes? The fudge factor of actress strength normally included for tolerances and temperature variation more than compensates for this small-scale deviation from perfection.

Putting magnets in a brushless motor? The steel rotor and stator guide the magnetic flux anyhow, so small errors in magnetization angle don't change the results much at all.

It's an interesting thing to think near, but usually doesn't bear upon too much.