Whenever current travels through a conductor, a magnetic field is generated, a fact famously stumbled upon by Hans Christian Ørsted around 1820. Depending on the shape of the conductor, the contour of the magnetic field will vary. If the conductor is a wire, however, the magnetic field always takes the form of concentric circles arranged at right angles to the wire. The magnetic field is strongest in the area closest to the wire, and its direction depends upon the direction of the current that produces the field, as illustrated in this applet.
Presented in the tutorial is a straight wire with a current flowing through it. Plus and minus signs indicate the poles of the battery (not shown) to which the wire is connected. The conventional direction of current flow is indicated with a large, black arrow. (As convention dictates, the current flow opposes the actual direction of the electrons, illustrated in yellow). The magnetic field lines generated around the wire due to the presence of the current are depicted in blue. To observe the direction of the field at any given point around the circumference of the wire, click and drag the compass needle, (its north pole red, its south pole blue). The direction of the magnetic field around the wire is also indicated by the small arrows featured on the individual field lines. Click the Reverse button to change the direction of the current flow and observe the effect this change exerts on the wire’s magnetic field.
There is a simple method of determining the direction of the magnetic field generated around a current-carrying wire commonly called the right hand rule. According to this rule, if the thumb of the right hand is pointed in the direction of the conventional current, the direction that the rest of the fingers need to curl in order to make a fist (or to wrap around the wire in question) is the direction of the magnetic field.