Use the Maxwell's screw rule. Very simple:



If a right-handed screw is turned so that it moves forward in the same direction as the current (from + to -), its direction of rotation will give the direction of the magnetic field (North end).

So if making a compass with a battery, a needle and a coil of insulated wire and you want the sharp pointy end of the needle to point north, then from this diagram;



you would put the sharp pointy end of the needle, pointing in the direction of the coil attached to the positive terminal on the battery.

Just checked my EDC flashlight magnet polarity and the makeshift compass performance:



A very flat neodymium magnet from an old HDD is epoxied to the back of the Photon REX (to magnetically mount it anywhere under the car hood, inside a computer case, and to other magnets sewed in my vest, hat, cap, gloves, etc.). It's orienting the On switch to the north in 3 seconds! And very reliably!

I am loving this thread!

-Blast

Looks like the middle drawing from the article graphic is incorrect then as the battery polarities are the wrong way around.

Yep. I've told you...

For me, knowing Maxwell's Right-Hand Rule doesn't help much. It may seem superficially simple because there are only two possible answers. It's hard because there are so many binary elements that can potentially be flipped:

• Wire coiled clockwise or anti-clockwise?
• Clockwise when looking from above, or clockwise when looking from below?
• Current flowing top to bottom or bottom to top?
• Are we talking conventional current flow (ie, positive to negative) or actual electron flow (the opposite)?
• Is the north end the end that seeks north, or the end which has the same polarity as the north pole?
• Left or right-handed screw (or hand, in some accounts)? (Maxwell has both kinds of rule, one for generators and one for motors.)
• Whether the needle picks up the same polarity as the coil, or the opposite polarity?

Knowing a bit more about the subject arguably makes it harder to remember. The difficulty is forgetting the possible accounts/diagrams which are wrong. I think I now know the answer, but I'll probably have forgotten it within a week, because it is all just too arbitrary to keep in my mind. (And further elucidation probably won't help and isn't requested.)

Of those three figures, I think (1) is correct and (3) is wrong. (In (2) I find it hard to tell whether the coil is clockwise or anti-clockwise.) It's interesting that he started with some correct physics and then got it wrong in the construction. It shows how hard it can be to keep track.

A maxim about 3D rendering programs seems relevant here: all correct renderers contain an even number of polarity errors.

Its only schoolboy physics, so it is not really that difficult to get right.

As long as you remember a few conventions.

Rule 1) - Current flows from the positive to the negative battery terminal.

Rule 2) - Magnetic flux lines flow from North to South.



This diagram is useful to remember as is shows a sectional diagram of a magnetic coil. Each circle with a dot in the centre shows current coming towards you out of the diagram and each circle with a cross shows current going away into the diagram. This is just a simplified interpretation of Maxwell's right hand rule.

The magnetic flux lines now indicate the magnetic south polarity is on the left hand side and the magnetic north polarity is on the right hand side i.e. as above in Rule 2.

Now that the north south polarity of the coil has been established from first principles, all that is required now is to determine which direction the sharp pointy end of the needle ends up. Will it be at the North or the South end of the coil.

This is pretty simple; for the sharp pointy end of the needle to point north in the earths magnetic field then the sharp pointy end of the needle has to be magnetised as a south pole. So therefore the sharp pointy end needs to pointing to the south pole of the coil i.e. the left hand side of the above diagram.

Of course if folks cannot tell their right hand from the left hand (you'll be surprised how many youngsters can't) then knowing the above information won't make much difference.

It is complicated, indeed. You've made a mistake too. The North Pole (geographic) of the Earth is actually the South pole of the Earth magnet (that's why the North arrow of the compass is attracted to that direction along the magnetic lines). So, the pointy end of a needle must go to the right on your image.



The problem with the screw rule is that people don't understand the whole picture - how the magnetic field line in particular point of space is forming a closed loop.

Anyhow, it seems to me that if a person is Okay with the simple logic and knows the Maxwell's rule, he/she will figure out what to do with all of that stuff eventually

Looking over these last four posts, I am really glad I can find the North Star reliably.....