Deflecting 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 by utilizing the various effects of electric current or voltage. The deflecting torque causes the moving system to move from its zero position. The deflecting torque is produced by utilizing one or more of the following effects of current or voltage:

• Magnetic effect ——————————- Moving-iron instruments.
• Electrodynamic effect ———————- (i) Moving coil instruments, (ii) Dynamometer type.
• Electromagnetic induction effect ———Induction type instruments.
• Thermal effect ———————————Hot-wire instruments.
• Chemical effect ——————————–Electrolytic instruments.
• Electrostatic effect —————————-Electrostatic voltmeters

The table below gives information about the electrical measuring instruments in which deflecting torque is produced by utilizing the first three effects.