RRB JE Electrical One Liner Questions and Answers series Part - 4 (Measurement)

 Measurement is a method of finding out value of physical quantities with the help of pointer deflection or null pointer or digital display and instruments used for the purpose are called Measuring Instruments.

Measurements are of two types absolute or primary Measurement and secondary Measurement. In absolute Measurement, measurement is done in terms of true value of physical quantities and these types of instruments are used in laboratories for calibration purposes example : Galvanometer 

Secondary instruments : All practical and domestic application instruments lies in this category examples : Multimeter,  Ohmmeter,  Ammeter etc.

1. Three types of torque are used in measuring instruments deflection torque,  damping torque and controlling torque.

2. There are different methods of producing deflection torque like electromagnetic induction, thermal effect etc. Main task of deflection torque is to move the pointer.

3. Controlling torque is produced by spring to stabilze the pointer. The spring used for Controlling must have low specific resistance,  low temperature coefficient of resistance, non magnetic and not subjected to fatigue.

4. Damping torque is used to damp out pointer oscillations. Damping device must be a good insulator, non evaporating,  non corrosive and least effected with temperature.

5. Permanent magnet moving coil (PMMC) type instruments  are used to measure DC voltages and currents. 

6. In PMMC deflection torque is directly proportional to charge/ current because of which it has a uniform scale.

7. Permanent magnet moving iron (PMMI) type  instruments are used to measure both AC and DC voltages & currents.

8. In PMMI instruments deflection torque is proportional to square of charge / currents  because of which it has non uniform scale.

9. In PMMC type  instruments eddy current damping is used.

10. In PMMI type instruments Air friction damping is used.

11. PMMC type instruments are used in uniform scale voltmeter, Ammeter,  Ohmmeter and multimeters. Multimeter uses a Rectifier circuit to convert AC into DC.

12. PMMI type instruments are used in non uniform scale voltmeters and Ammeters.

13. Ohmmeter is used to measure resistance of medium ranges.

14. Kelvin double Bridge method is used for low resistance measurement.

15. Wheat stone Bridge and Corey Foster's  Bridge method is used for medium resistance measurement.

16. Loss of charge method is used for high resistance measurements. 

17. Megger is used  to measure insulation resistance of electrical devices, cables and Electronic devices. Megger is a combination of DC generator and ohmmeter. It produces 100 V voltage and few microampere of currents to measure the resistance.

18. Voltmeter is used to measure voltage. Range of voltmeter can be increased by using  multiplier in series which is a high value resistance.

19. Ammeter is used to measure current. Range of Ammeter can be increased by using shunt in parallel which is a low value resistance.

20. Multimeter can measure AC and DC voltage, current and resistance.

21. Maxwell's  Bridge is used to measure inductance of low Q value coils.(Q<10)

22. Hay's Bridge is used to measure inductance of high Q value coils.(Q>10)

23. Anderson's Bridge is used to measure inductance of coils fastly and accurately.(Q<1)

24. Heaviside 's Bridge is used to measure inductance having low Q value.

25. De Souty 's Bridge is used to measure capacitance.

26. Schering Bridge is used to measure  capacitance and relative permittivity.

27. Wein 's Bridge is used to measure capacitance and frequency.

28. LCR meter is used to measure resistance,  inductance and capacitance.

29.  Pyrometer is used to measure temperature of train axle boxes, motors and transfromers etc. Pyrometer works on Stefan Boltzmann law principle which tells that energy radiated by an heating object  is directly proportional to forth power of its absolute temperature.

30.  Techometer is used to measure speed of train. It mainly measures shaft speed of dc series motor.

31.  Odometer is used to measure speed of vehicles used by police.

32. RADAR is also used to measure speed and direction of any object. RADAR is an acronym for radio detection and ranging.

33.  SONAR is used to locate objects beneath the sea. Sonar is acronym for sound navigation and ranging.

34. Potential Transfromer or PT is used to measure very high voltages. It is a step down transformer which transfroms high Voltage into low voltage of 120 V range.

35. Current Transfromer or CT is used to measure very high currents. It is a step up transformer which steps up the low voltage high current into high voltage low current of 5 ampere range.

36. Anemometer is used to find out the force of wind.

37. Hygrometer is used to measure humidity of air or water vapour present in air.

38. Barometer is used to measure atmospheric pressure. 

39. Hydrometer is used to measure specific gravity of liquids.

40. PMMC type instrument can be used as fluxmeter by eliminating control spring.

41. The chemical effect of current is used in DC ampere hour meter.

42. In megger controlling torque is provided by coil.

43. Shunt resistances are made of magnin.

44. In eddy current damping disc or former is made up of material that is a conductors  but non magnetic examples copper and aluminium.

45. Hot wire instruments and PMMI type instruments have square scale.

46. The range of PMMI is extended by changing number of turns of operating coil.

47. Electrostatic instruments are used for voltage measurement.

48. In a single phase energy meter braking torque is provided by permanent magnet.

49. Luxmeter is used for illuminance measurement.

50. A wire gauge is a device used to measure diameter of wire.

Author:- Amarjeet Singh Jamwal authored articles on Basic Electronics EngineeringElectric Traction, and Electronics Practical for INFO4EEE Website

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