Arrangement : A straight wire AB is connected to a battery V and key K. The wire is held horizontally over a magnetic needle.
Working : When key is closed, current flows in the wire in the direction as shown. The needle deflects one side. When key is taken out and current in wire becomes zero, needle returns back to its initial position (S – N). This shows that a magnetic field is associated with an electric current.
When direction of current in wire is reversed, direction of deflection of needle is also reversed. If direction of current be kept same and wire be put under the needle, direction of deflection of needle again becomes reversed.