(i) A set-up for plotting the magnetic lines of force in a straight wire is shown in the diagram. A smooth cardboard with a hole at the centre is placed horizontally and a wire passes vertically through the hole. Some iron filings are sprinkled on the card-board and an electric current is passed through the wire. On slightly tapping the card-board the iron filing arrange themselves in concentric circle around the wire, as shown in the diagram. The direction of magnetic lines of force is clockwise since the the current in the straight wire is flowing downwards. If the current flow is upwards then the direction of lines of force is anti-clockwise. (See diagram (a) and (b))
(ii) Arrangement of the apparatus is the same, as in (a), only the wire is in the form of a circular coil and is passing through two holes, A and B in the cardboard. On passing the current through the circular coil, iron filings sprinkled on the horizontal surface of the smooth cardboard make a pattern of the lines of force, as shown in the diagram.
One face of the coil has north polarity while the other face shows south polarity, in accordance with the following rule: Looking at one face of the coil, if the direction of the current is anti-clockwise, that face has north polarity and the opposite face has south polarity.
(iii) The accompanying diagram illustrates the pattern of magnetic lines of force in a straight solenoid. As shown in the diagram, the solenoid behaves like a cylindrical magnet having North and South poles at free ends. If the current flows in the reverse direction, the polarity at the free ends is also reversed.
