Determine about the rod in magnetic coil
the rod is placed in a magnetic coil deliver alternating current, the rod suffers a change within length for each half cycle of alternating current. It is, the rod vibrates along with a frequency twice which of the frequency of A.C. The amplitude of vibration is generally small, but when the frequency of the A.C. coincides along with the natural frequency of the rod, in that case the amplitude of vibration increases because of resonance.