Direction of compass needle nearby an electric wire

Question

Let through a straight wire flow an electric current. Using a compass needle how the needles tip will be directed to the wire? What happens when we switch the current direction and approach the needle again to the wire?

To get a source will be nice, but nicer it will be to get an answer from experiment which unfortunately I didn't in the past and not able to do at the moment.

A Google image search will find you pictures illustrating this — John Rennie, Aug 21 '14 at 10:43
@john rennie Yes, this is ok. Does this is opposite to your answer about symmetry in this question? For me the needles direction change if you change the current is pure asymmetric. Without experiment and derived therefrom rules you can't predict the result. — HolgerFiedler, Aug 21 '14 at 11:06
Yes, as already pointed out to you in your other question, the direction arises from a rule/convention. It can go one of two ways and we make a decision that the universe will obey a right-handed convention when we evaluate a vector product. — ProfRob, Aug 21 '14 at 13:30
@RobJeffries Could you write this down as an answer in the older question, please. — HolgerFiedler, Aug 21 '14 at 15:14

ProfRob · Accepted Answer · 2014-08-21T16:37:45.533

I doubt my description can be improved other than with a picture.

Direction of a compass needle caused by a flow of current

Now the interesting thing here is that I do not know which way the current is flowing, because I do not know which way the compass needles are colour-coded. I guess that the red represents the north pole. I do know that if the current flow is reversed then the needles will swap direction.

It really doesn't greatly matter. There is a defined direction and it is a convention that arises from the definition of vector product, the definition of conventional current flow and the sign convention in Ampere's law relating the curl of the B-field to the current density.

Direction of compass needle nearby an electric wire

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