There is no fundamental difference between the operating principles of ammeters and voltmeters. Both are current-operated devices (except electrostatic type voltmeters) i.e. deflecting torque is produced when current flows through their operating coils. In an ammeter, the deflecting torque is produced by the current to be measured or by a definite fraction of it whereas […]

Moving Coil Instrument Working Principle  A moving coil meter is a very commonly used form of an analog instrument because of its sensitivity, accuracy, and linear scale, although it only responds to d.c. signals.  The simple view of the construction of the moving coil instrument is shown in the figure. It consists of a powerful

The moving iron instruments or MI instruments are quite cheap in cost, simple in construction and very accurate at fixed power supply frequency. These instruments can be used on AC as well as DC. Therefore, these are widely used in laboratories and on switching panels. These instruments are used either as ammeters or voltmeters only.

In order to ensure proper operation of indicating instruments, the following three torques are required: Deflecting Torque | Controlling Torque | Damping Torque  One important requirement in indicating instruments is the arrangement for producing operating or deflecting torque (Td) when the instrument is connected in the circuit to measure the given electrical quantity. This is achieved