# I Who defined the positive direction of the magnetic field?

Tags:
1. Feb 11, 2019

### Jairo Amaral

This is about the history of electromagnetism. I already know about the convention that defined which electric charge is the "positive" one, which ended up making the electron a negative particle.

But what about the "positive" direction of the magnetic field? Does anybody here know
how it was arbitrated, and who did it? Specifically, did it come before the equation relating it to electric charges? Or was the direction defined by the equation itself?

2. Feb 11, 2019

### Jeff Root

for it, but it is an interesting part of the story: The "north" end of a
magnetic compass was defined as the end of the needle pointing
toward Earth's north pole. When the mechanism of magnetism
was puzzled out, it was realized that opposites attract, so the
end of a compass needle marked "north" is actually what is
called "south" on all other magnets.

-- Jeff, in Minneapolis

3. Feb 12, 2019

### A.T.

No. The compass needle is marked just like other magnets. The Earth's magnetic poles are marked opposite to the usual convention.
https://en.wikipedia.org/wiki/North_Magnetic_Pole#Polarity

4. Feb 12, 2019

### Staff: Mentor

This can be tested by hanging a bar magnet on a string, balancing it at its center of mass so it rotates freely in a horizontal plane. It should end up with the "N" end pointing north.

5. Feb 12, 2019

### sweet springs

Say you know mass m and charge of the particle q and throw it into magnetic field with velocity $\vec{v}$. Motion of the particle will be decided by force $$\vec{F}=q\vec{v}\times\vec{B}$$. Observing the motion, you can decide direction and magnitude of B. This is one of ways to let you know the conventional direction of magnetic field.

6. Feb 12, 2019

### Jairo Amaral

Hi! This is the convention. I'm asking how it appeared.

Was it like "I arbitrate the positive direction is that way, and now will derive an equation to agree with it", or was it like "I arbitrate the equation is thus, and now we'll use it to find the directions"?

7. Feb 12, 2019

### sweet springs

The equation gives us convention. You may use F= - qvXB if it is shared with all the other people. Why plus not minus sign is a matter of science history that I am not good at.

Last edited: Feb 12, 2019
8. Feb 12, 2019

### Klystron

Please relate who you think defined the electrical polarity convention. In USA we learn it was Benjamin Franklin but I have read that other countries make different historical (and ideological) claims.
My understanding, probably from reading Isaac Asimov and other science historians, is that J. F. C. Gauss and W. Weber developed, or at least confirmed, polarity conventions while inventing electrodynamics.

Franklin and Gauss were both such prolific inventors in so many areas, historians may give them precedence from familiarity. I can look for definitive sources specifically for magnetic polarity, if this is important.

9. Feb 12, 2019

### Jairo Amaral

I read it was both Benjamin Franklin and William Watson. I didn't know there was more than one version.

It's neither work nor formal study, so it isn't urgent. But I plan to write about science for lay people in the future, so it'd be nice to know. It it's too much trouble for you to get the exact answer, could you indicate me books that might have this? I'd love to read them anyway.

10. Feb 12, 2019

### Jairo Amaral

So we'll all discover that, and we'll give it enough publicity so it isn't forgotten again.

11. Feb 13, 2019

### Mister T

There is no positive direction involved here. The direction a compass needle points, by definition, is called north.

Who came up with that naming convention? My guess would be Sir William Gilbert. You should be able to verify that with a google search.

12. Feb 13, 2019

### Jairo Amaral

Even though it started with just north and south, at some point a positive direction was established, since we're using it right now. And the positive direction of the magnetic field points to the south pole of the magnet. I'm trying to find out how it all started.

13. Feb 14, 2019

### sweet springs

Frequent geomagnetic reversal is observed in geological history. Some thousand or millions year past or future our current convention N would point out the south and S would point out the north. Further confusion is that compass painted N is polar S, painted S is polar N

Electric dipole qd is usually modeled as plus charge at z=+d/2 and minus charge at z=-d/2
Similarly magnetic dipole $q_m d$ is composed of plus magnetic charge at z=+d/2 and minus magnetic charge z=-d/2. The magnetic dipole corresponds to the Earth. z=d/2 corresponds North pole and z=-d/2 corresponds South pole.

Last edited: Feb 14, 2019
14. Feb 14, 2019

### A.T.

No, it's not. See post #3.

15. Feb 14, 2019

### Mister T

What do you mean by your claim that you're using it right now? Can you give us some context?

16. Feb 14, 2019

### Mister T

Likewise, one can place a bar magnet on top of a slab of styrofoam or the like, and float it on the surface of a tub of water. The end labelled "N" will be attracted to the ends of other bar magnets labelled "S". There is a south magnetic pole near Earth's geographic north pole.

17. Feb 14, 2019

### Jairo Amaral

In the equations for electromagnetism, the magnetic fields have positive and negative directions, not north and south directions.

18. Feb 14, 2019

### Staff: Mentor

Once you've chosen a convention for which is the positive charge and which is the negative, it's almost reflexive to choose the convention for the magnetic field direction such that the force is $q\vec{v}\times\vec{B}$ instead of $-q\vec{v}\times\vec{B}$ - why introduce a gratuitous minus sign?

But note that there's yet another arbitrary convention here - the right-hand rule for the cross-product.

19. Feb 14, 2019

### jbriggs444

It is not plus versus minus. That would be too easy. It is $qv \times B$ versus $B \times qv$. The physics does not dictate the order in which we place terms in a formula. [Which is to restate @Nugatory's point in different words]

20. Feb 14, 2019

### Jairo Amaral

That's what I'm talking about. If the direction of the magnetic field was defined first, then it's just a matter of placing the cross product in the convenient order.