Advantages
- The scale is
uniformly divided
- The power
consumption can be made very low (25μW to 200μW).
- The
torque-weight ratio can be made high with a view to achieve high accuracy.
- A single
instrument can be used for multi range ammeters and voltmeters.
- Error due to
stray magnetic field is very small.
- Very accurate
and reliable.
- No hysteresis loss as the magnetic flux is practically constant.
Disadvantages
- They are suitable for direct current only.
- The instrument cost is high.
- Variation of magnet strength with time.
- About 50% more expensive than moving iron instruments.
Sources of Errors
- Frictional error.
- Magnetic decay.
- Thermoelectric error.
- Temperature error.
Reduction of errors
- Proper pivoting
and balancing weight may reduce the frictional error.
- Suitable
aging can reduce the magnetic decay.
- Use of
Manganin resistance in series (swamping resistance) can nullify the effect
of variation of resistance of the instrument circuit due to temperature
variation.
- The stiffness of spring, permeability of magnetic core (Magnetic core is the core of electromagnet or inductor which is typically made by winding a coil of wire around a ferromagnetic material) decreases with increases in temperature.
