Least expensive instrument for DC measurements is
Right Answer is:
Attraction type moving instrument
The least expensive instrument for DC measurements is an attraction-type moving instrument.
Moving iron instruments are of two types
- Attraction Type
- Repulsion Type
Attraction Type Moving instruments— These instruments are based on the principle that when an unmagnetized soft iron piece is placed in the magnetic field of a coil then the piece is attracted towards the coil.
Repulsion type Moving instruments— These instruments are based on the principle of repulsion between the two iron pieces similarly magnetized. Since deflection θ is proportional to the square of the current through the coil, therefore scale of such instruments in non-uniform being crowded in the beginning.
Advantages of Moving Iron Instrument
The various advantages of moving iron instruments are,
1) The instruments can be used for both a.c. and d.c. measurements.
2) The torque to weight ratio is high, errors due to the friction are very less.
3) A single type of moving element can cover a wide range hence this instrument; are cheaper than other types of instruments.
4) There is no current-carrying part; in the moving system hence these meters are extremely rugged and reliable.
5) These are capable of giving good accuracy. Modern moving iron instruments have a d.c. an error of 2% or less.
6) These can withstand large loads and are not damaged even under severe overload conditions;
7) The range of instruments can be extended.
Disadvantages of Moving Iron Instrument
The various disadvantages of moving iron instruments are,
1) The scale of the moving iron instruments is not uniform and is cramped at the lower end. Hence accurate readings are not possible at this end.
2) There are serious errors due to hysteresis, frequency changes, and stray magnetic fields.
3) The increase in temperature increases the resistance of the coil, decreases the stiffness of the springs, decreases the permeability, and hence affects the reading severely.
4) Due to the non-linearity of the B-H curve, the deflecting torque is not exactly proportional to the square of the current.
5) There is a difference between a.c and d.c calibrations on account of the effect of the inductance of the meter. Hence these meters must always be calibrated at the frequency at which Hey are to be used. The usual commercial moving iron instrument may be used within its specified accuracy from 25 to 125 Hz frequency range.
6) Power consumption is on the higher side.