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Materials used for Precious Works

In some precious works associated with electrical engineering such measuring instruments, gauges etc. used in laboratories and research works, the accuracy of results or measured values indicated by the measuring instruments or gauges, is the first priority. Cost of instrument is not a matter. To increase the accuracy of measuring instrument, it becomes very important to select a proper and suitable material for designing and making these precise measuring instruments. The material used for precious measuring instruments should have following properties –
  1. Suitable resistivity as per required operation or application.
  2. Minimum temperature coefficient of resistance (ideally zero).
  3. Minimum coefficient of thermal expansion ( ideally zero ).
  4. Free from aging effect.
  5. High corrosion resistance or free from oxidation at high operating temperature.
  6. Suitable mechanical strength to withstand with mechanical stresses developed during operation.
  7. No effect of temperature fluctuations.

Some of the materials used for precious work associated with Electrical Engineering are listed below-

  1. Manganin
  2. Constantan

Manganin

Manganin is an alloy of copper, manganese and nickel. The feature of manganin is that its resistivity remains constant over wide range of temperature. Manganin is having very low temperature coefficient of resistance ( almost zero ). Which make is suitable for standard resistances used in measuring instruments.

Composition of Manganin

Properties of Manganin

  1. Resistivity at 20oC: 43-48 µΩ -cm.
  2. Temperature coefficient of resistanceat 20oC: 0.00001 /oC.
  3. Melting point: 960oC.
  4. Specific gravity: 8.4 gm /cm3.
  5. High resistance to oxidation.
  6. Good mechanical strength.
  7. Good stability with temperature and mechanical stress.

Use of Manganin

  1. Manganin is best material for making electrical connections for measuring instruments such as shunt resistors ( used in ammeters ), series resisters ( used in voltmeters ), swamp resistors ( to reduce thermal Emfs ) etc.
  2. For making standard resistors.
  3. Sometimes manganin wire is used as an electrical conductor in cryogenic systems, minimizing the heat transfer between points which needs electrical connections.

Constantan

Constant is copper nickel alloy. Like manganin, it also having very low temperature coefficient of resistance ( slightly higher than manganin ). Therefore, its resistivity is also remains constant over wide range of temperature. Which make it suitable to use for making standard resistors and measuring instruments and gauges.

Composition of Constantan

Properties of Constantan
  1. Resistivity at 20oC : 50 µΩ -cm.
  2. Temperature coefficient of resistance at 20oC: 0.00001 /oC.
  3. Melting point: 1300oC.
  4. Specific gravity: 8.9 gm /cm3.
  5. High resistance to oxidation or free from oxidation at operating temperature.
  6. Good mechanical strength, tensile strength and ductility.
  7. Good stability with temperature and mechanical stress.

Use of Constantan

  1. Constant is also used for making electrical connections for measuring instruments such as shunt resistors ( used in ammeters ), series resisters ( used in voltmeters ), swamp resistors ( to reduce thermal Emfs ) etc.
  2. For making standard resistors.
  3. Constant is being used as gauge alloy since very old time. Constantan is having very high strain sensitivity or gauge factor, very low temperature coefficient of resistance, good fatigue life and high elongation capability, which make it suitable for gauges.
  4. For making heating elements used for precious temperature control.

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