Question

AC Frequency is measured by sonometer in detail

Answer 1

A sonometer is an apparatus which used to study transverse vibrations of stretched strings. It is in the form of a hollow wooden rectangular box. On the wooden rectangular box there are two bridges along with a pulley at one end. Wire string is attached to one end  of the wooden box which run over the bridges and pulley carries a weight hanger at the free end.A sonometer is used to determine the frequency of alternating current. A step down transformer is used for the finding the frequency of A.C. because the voltage of the A.C. mains is 220V, which is very dangerous. What step down transformer does is that it reduces this voltage to 6 volts.The string wire of the sonometer is a non-magnetic metallic wire like brass or copper. A horse shoe magnet is placed at the middle of the sonometer wire so that the magnetic field is applied perpendicular to the sonometer wire in a horizontal plane. When an alternating current of definite frequency passes through the wire there will be interaction between the magnetic field and the current carrying conductor. So a force will act on the conductor in a direction perpendicular to both the field and the direction of current. 
When A.C. is passes through the conductor, the direction reverses periodically, the direction of force also reverse periodically and hence, the conductor vibrates. Since the current flowing is alternating, the wire vibrates with a frequency equal to the frequency of A. C. By adjusting the length of the vibrating wire segment, this frequency can be made equal to the natural frequency of the wire segment. Then the resonance takes place and the wire vibrates with maximum amplitude. At this stage, the length of the wire segment is called the resonating length and it increases with increase in the mass of the suspended weights.