Which of the following instruments does not use the effect of current for measurement purposes?
Right Answer is:
Electrostatic Instrument does not use the effect of current for measurement purposes.
These are the only instruments that are directly voltage-sensitive and do not depend on a current for their operation. Electrostatic instruments are almost always used as voltmeters and that too is more as a laboratory rather than as industrial instruments.
The operating principle of an electrostatic instrument depends on the force of attraction between two or more electrically charged conductors between which a potential difference is maintained, and this force gives rise to a deflecting torque. The electrostatic mechanism resembles a variable capacitor, where the force existing between the two parallel plates is a function of the potential difference applied to them.
Basically, there are two types of electrostatic (also known as electrometers instruments):
(i) Quadrant type
(ii) Attracted-disc type
Advantages of Electrostatic Instruments
The following are the advantages of electrostatic instruments:
- They can be manufactured with a very high accuracy.
- They can be used on either a.c. or d.c. and over a fairly large range of frequencies.
- The instruments may be calibrated with d.c, and yet the calibration would be valid for a.c. also since the deflection is independent of the waveform of the applied voltage.
- Since no iron is used for their construction, they are free from hysteresis, eddy current losses and temperature errors.
- Their power loss is negligible.
- They are unaffected by stray magnetic fields although they have to be guarded against any stray electrostatic field.
- Once the discs are charged, no more current is drawn from the circuit and the instrument represents infinite impedance.
- They can be used upto 1000 kHz frequency without any serious loss of accuracy.
- They do not draw any continuous current on d.c. circuits and that drawn on d.c. circuits are extremely small. Hence, such voltmeters do not cause any disturbance to the circuit to which they are connected.
Limitations or Electrostatic Instruments
The following are the limitations of electrostatic instruments:
- Their use is limited to certain special applications, particularly in a.c. circuits of relatively high voltage where the current taken by other instruments would result in erroneous indications.
- Low voltage voltmeters are liable to friction errors.
- Since the deflecting torque is proportional to the square of the voltage, their scales are not uniform although some uniformity can be obtained by suitably shaping the quadrants of the instruments.
- They are expensive and are not likely to be durable.
- They are, inherently, laboratory-type rather than industrial-type instruments.