Magnetic Effects Of Electric Current – When current passes through a conductor, a magnetic field is produced in the space around it. If the conductor is a straight wire, the lines of force of this magnetic field would be in the form of concentric circles (fig. 17.1). These lines of force can be traced on a piece of cardboard with the help of a compass needle. 
The direction of the lines of force can be determined by the right hand rule according to which if we grasp the current carrying conductor in our right hand with the thumb being stretched in the direction of current, the fingers would curl in the direction of lines of force (fig. 17.2). This rule shows that if the current is flowing through wire from bottom end to top then the direction of lines of force would be anticlockwise (fig. 17.3-a). On the contrary if the current is flowing from top towards the bottom, it would be clockwise (fig. 17.3-b).
A coil of few turns of wire is shown in fig. 17.4.its two ends are connected with a battery so that a current is passing through it. In fig. 17.4(b) vertical cross section of a single turn of this coil has been shown by a small circle. The direction of current has been indicated by placing a dot or a sign of multiplication in these circles. A dot indicates that the current is directed out of the plane of the paper, i.e., it is flowing towards us where as a sign of cross (x) would mean that the current is directed into the paper i.e it is flowing away from us.
(Fig. 17.4b) shows the magnetic lines of force generated by the flow of current in this coil. These lines are straight and parallel in a small region near the centre of the coil and are directed perpendicular to its plane. They are approximately circular near the wire. Thus the field is uniform in a small area surrounding the centre of the coil. In the remaining portion it is non-uniform. The direction of the field, i.e, that of the lines of force can be determined by the right hand rule.
Magnetic Field of a Current Carrying Solenoid.
A solenoid is a closely wound cylindrical coil of insulated wire (fig. 17.5a) shows the magnetic lines of force generated by the passage of current through a solenoid. These lines emerge out from one end of the solenoid and after encircling around, enter into it through the other end. Inside the solenoid, the lines of force are parallel and all point in the same direction. It can be seen that these lines resemble the pattern of the lines of force due to a bar magnet. On the basis of this resemblance we can say that one end of the solenoid behaves like a north pole and the other, like a south pole. As the lines of force emerge out of the north pole of a magnet and enter into it through the South Pole, therefore in fig. 17.5 (b), the end B of the solenoid would be North Pole and end A would be a south pole. The polarity of a current carrying solenoid can be determined by the following rule.
Hold the solenoid in your right hand by curling the fingers in the direction of the current; the stretched thumb would indicate towards the North Pole (Fig. 17.6).
The poles of a solenoid can also be found out by the following easy rule.
Hold down the end of the current carrying solenoid in front of you, if the direction of current flow through this end is anticlockwise, it would be North Pole; otherwise it would be a south pole.