 The magnetic field for magnets comes out of the north pole and goes into the south pole. A current also generates a magnetic field, as we've previously discussed. But there is no north or south pole in this case. So how do we find the direction of the circular magnetic field around a current? Esther discovered the magnetic field around a current. But it was Ampere that described the direction of the magnetic field. He said that if one were to imagine a little mannequin to swim in the direction of the current and to face the needles, then the north pole of the needle would be the direction of the left arm and the south pole that of the right arm. This description is slightly hard to follow. So these days we use the right hand rule to identify the direction. The right hand rule is called the right hand rule because you use your right hand. So you put your thumb in the direction of the current and you curl your fingers around the current. The direction which your fingers curl tells you the direction of the magnetic field. So let's say you had a current carrying wire and in it there is a current going towards the left. So you curl your fingers around the wire and the direction which your fingers curl will show you the magnetic field. So in this case, on top of the wire the magnetic field points out of the screen towards you. In front of the wire the magnetic field points down. Underneath the wire the magnetic field points towards me and behind the wire the magnetic field points up. Because magnetic fields have direction, if we look at a current going up the magnetic field goes into the screen on the right hand side of the wire and comes out of the screen on the left hand side of the wire. How we draw this is by using crosses to indicate that the magnetic field goes into the screen. This can be thought of as an arrow going away from you and dots to indicate a magnetic field coming out of the screen an arrow going towards you. Now that we have the direction of the magnetic field at a point we can also find the strength of the magnetic field due to a current at that point. Just like electric fields, the strength of the magnetic field increases the closer you are to the current and decreases further away. The stronger the current, the stronger the field. So applying Amper's law we get this formula describing the magnetic field strength. b equals mu naught times i divided by 2 pi r where b is the strength of the magnetic field in Tesla's mu naught is a constant. It is 4 pi times 10 to the power of negative 7 also known as the permeability of free space and measured in Tesla meters per amps. i is the strength of the current measured in amps and r is the perpendicular distance from the current measured in meters. The formula shows the strength of the magnetic field increasing as the strength of the current increases. This fits with our expectations from the experiment. The magnetic field strength also increases as distance from current decreases which also fits with the experiment. So this formula behaves the way we want it to.