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Materials used for Rheostats

Rheostats are variable resistors or adjustable resistors. These are used to divide the voltage or control the current through circuit. Rheostats are often used as power control devices, for example to control the speed of electric motor, to control light intensity (dimmer), to control the temperature in electric oven and furnaces.
Rheostat is represented in an electric circuit by symbol given in figure below- symbols of rheostats Many rheostats are wire-wound and have a lengthy conductive material wire wrapped in spiral form on an insulating hollow cylinder. A typical wire-wound rheostat is shown in figure below-

wire-wound rheostat Generally, the wire-wound rheostat is designed considering one volt per turn maximum. Materials used for making rheostats are high resistivity conducting materials. However, the material may have large thermo-emf and a large value of temperature coefficient of resistance. But the material should meet to some special requirement such as high permissibly working temperature and low cost. As the material is material is required in bulk quantity have the cost of material becomes very important.

Properties Required in Material Used for Rheostats

  1. High resistivity.
  2. High working temperature.
  3. High corrosion resistance.
  4. Suitable mechanical strength.
  5. Suitable ductility to enable to make the wires.
  6. Low cost.

List of Materials Used for Rheostats

Following materials are used for rheostats-
  1. Platinum
  2. Constantan

Platinum

Platinum is a chemical element. It is having the chemical symbol Pt and atomic no. 78. Platinum is least reactive metal. It has remarkable resistance to corrosion, even at high temperature. Therefore it is considered as noble metal.Platinum is a precious metal, it is very popular for making jewelry.

Properties of Platinum

  1. Resistivity: 10.50 µΩ -cm.
  2. Temperature coefficient of resistance: 0.003927 /oC.
  3. Melting point: 1768.30oC.
  4. Specific gravity: 21.45gm /cm3.
  5. High resistance to oxidation.
  6. High ductility.
  7. Highly malleable.
  8. Good mechanical strength.
  9. Good stability with temperature and mechanical stress.

Use of Platinum

  1. Platinum is an incredible material with high resistivity and melting point. It is very much suitable for electrical heating elements, rheostats. But due to very high cost, its use in electrical engineering is limited to laboratory furnaces with a working temperature of 1300oC, rheostats, and resistance thermometers.
  2. Platinum is a precious metal, it is very popular for making jewelry.
  3. In medical platinum is used in chemotherapy for treatment of certain types of cancers.

Constantan

Constant is a 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.

Composition of Constantan

Properties of Constantan

  1. Resistivity at 20oC: 50 µΩ -cm
  2. Temperature coefficient of resistanceat 20oC: 0.00003 /oC.
  3. Melting point: 1300oC.
  4. Specific gravity: 8.9gm /cm3.
  5. High resistance to oxidation.
  6. Good mechanical strength.
  7. Good stability with temperature and mechanical stress.

Use of Constantan

  1. Constant is used for making electrical connections in instruments such as, shunt resistors (used in ammeters), series resisters (used in voltmeters), swamp resistors (to reduce thermal emf) etc.
  2. For making standard resistors.
  3. For making rheostats.
  4. 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 gauge .
  5. For making heating elements.

Compression of Materials used for Rheostats with Respect to Required Properties for Rheostats

Sl No.PropertyPlatinumConstantan
1Melting point (oC)1768.301300
2Resistivity (µΩ-cm) at 20oC10.5050
3Oxidation ResistanceHighHigh
4Thermal coefficient of expansion (/K)9.0 x 10-618.8 x 10-6
5Temperature coefficient of resistance (/oC) at 20oC0.0039270.00003




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