These instruments belong to the category of versatile instruments. These instruments are generally used for measurement and record of process variables such as vacuum, pressure, angle of rotation, flow etc. The basic principle employed here is null balance principle. Differential amplifiers, induction transducers, ferrodynamic converters etc. are used here in an effective way. Supporting instruments used here are balancing motor, linkage and chart drive mechanisms. A ferrodynamic converter is composed of a powerful moving coil placed in an alternating magnetic field. The magnitude and phase of the e.m.f induced in the coil is proportional to the angle of rotation of the coil.
