A full-scale deflection of 90° is produced by a current of 10mA flowing
through the moving coil of d. c instrument. Determine the value of the
shunt resistance required to produce full-scale deflection with 12mA, the
resistance of the coil and springs being 1.2ohms. The constant of the control
spring is 3x20^-5 N-m/rad and the moment of inertia of the coil is 4x10^-7 kg-m². Assuming that the shunt produces the only available magnetic damping and that air damping is negligible derive an expression for, and
determine the value of, the periodic time of the coil system. Calculate the
ratio of the amplitude of consecutive swings.