Materials Used for Resistors:

Electrical Resistors         It is almost impossible to manufacture a circuit  component which has only one property i.e.,solely a pure resistance, an inductance for or a capacitance.For example, a resistor does not have resistance only but an associated inductance and capacitance, an inductor in addition to its inductance has resistance and inturn capacitance, and a capacitor has resistance in addition to capacitance. 

            You will know about materials used in resistors.The impurities present in a circuit component produce unwanted quantities called Residues. However, a particular quantity such as resistance in a resistor, inductance in an inductor, or capacitance in a capacitor is made to dominate in order that the effect of residues is very small and it is possible to design pure circuit components for specified limits of accuracy.As many of the visitors asked me the material used for making standard resistor I am writing this post.The below is the answer for the question standard resistance coils are made of which material and why?

 Must Read:
      
          Resistors and resistive networks are extensively used in instrumentation and measurement work.The foremost properties of materials used in resistors meant for precision work are stability or permanence with time, low resistance temperature coefficient, low thermoelectric emf with copper, high resistivity, resistance to oxidation, corrosion and moisture, ease of manufacture and low cost.

       No single material possesses all the above men-boned properties and, therefore only that material is selected which is best suited for the particular application involved.Standard resistor is made from the materials mentioned below:

The most widely used materials in precision resistors are : 

Manganin: 

          It is an alloy of copper, manganese and nickel used as resistance material for precision resistors and for resistance measuring apparatus. It has a nominal composition of 84 percent copper, l2 percent  manganese and 4 percent nickel. It has a resistivity of 0.45 to 0.50*10⁻⁶ Ωm (nearly 25 times that of copper) and develops a thermal emf of 2 to 3 μV/°C, against copper. When properly heat treated, it gives a stable resistance value with time i.e., it shows no ageing effects. 

        The foremost property of manganin is that it has almost a zero temperature coefficient of resistance near about room temperatures. A representative value may be taken as 0.004 percent per °C at 20°C. Manganin is used for resistances of very high accuracy when the temperature rise is not expected to rise above 15 to 20°C. 

Constantan:

         These are a series of alloys of nickel and copper containing 40 to 60 percent nickel, with a small amount of manganese to improve their mechanical properties.All these alloys exhibit similar electrical properties. They are sold as constantan, or under various trade names for use as thermocouple materials. They have thermoelectric emfs against copper of about 40 μV per °C. However, except for their large thermoelectric emf, the electrical properties of these alloys are remarkably similar to those of manganin. 

             Constantan has a resistivity at ordinary temperatures of about 25 times that of copper (about the same as manganin), is corrosion resistant, inexpensive, and easy to work. This can be easily soft soldered to copper.  It finds extensive use in cases where its high thermoelectric emf against copper is not a disadvantage.For example, this material is used in resistors designed for a.c. operation.It also finds application in resistors of 1000Ω and above as in voltmeter multipliers where the thermal emf generated at copper constantan junction is negligibly small as compared to the emf being measured.

Nickel Chromium alloys;

         These alloys have a somewhat higher temperature coefficient of resistance than that of manganin and constantan. Nichrome is an example of this class of alloys. These alloys cannot be used in precision resistances. Nichrome has a very high resistivity (about 50 times that of copper) and resists corrosion even at very high temperatures. It is often used in the rougher class of resistors, where small size is all important or where the operating temperatures are high. However, these alloys are difficult to solder. 

Gold chromium: 

           It is an alloy of recent introduction, which appears to be very promising for some applications. It is made with slightly over 2 percent of chromium. This alloy has a resistivity at room temperature of about 20 times that of copper. Its temperature coefficient can be made extremely small by baking it at fairly low temperatures. It has a thermoelectric emf of 7 or 8 μV per °C with copper. For many applications, the extremely small temperature coefficient of gold chromium alloys makes their use desirable, especially for heat resistant standards. 

Spools (Formers) for Resistance Coils:

         The spools (formers) made up of metals are commonly used for high quality d.c. resistors as compared with wooden spools used earlier. The advantages of metal spools are ;

(i) They do not absorb moisture from the atmosphere, unlike wooden formers. Therefore, they do not either expand or contract because of change, humidity. Therefore, no varying stresses are produced in the resistance coil which otherwise cause a change in their resistance value. 

(ii) The metal spools readily' dissipate heat produced on account of the flow of current through the resistance coil. This is because the resistance wire is in intimate thermal contact with the spool and heat is readily transferred to the spools from which it is dissipated to surroundings due to convection and radiation. The outside surface of the spool is already exposed to the surroundings and hence the entire surface of the spool i.e.,both inside and outside is effective in the dissipation of heat thereby preventing any hot spot temperatures to build up in the resistance coil.                   
             
             Metal spools are ordinarily made of brass.This is because brass has a coefficient of thermal expansion which is nearly equal to that of materials used for resistance coils ,and hence there is no significant differential expansion between the coil  and the spool.Therefore there are no significant  changes in  resistance wires on account of  changes in temperature.

              For a.c. applications, the use of metallic spools is undesirable, even out of question because of the eddy current losses produced in them. Ceramic is the universally used material for spools of a.c. resistors. The disadvantages of ceramic spools for use in resistors of high precision are their poor. thermal conductivity and their temperature coefficient of linear expansion which is much smaller than that of the resistance wire. This leads to differential expansion with changes in temperature leading to stresses produced in the windings.  

Resistance Wires:

The resistance wires are  generally made up of double silk or silk and cotton covered. The wire is enamelled before these coatings are applied. 

         High quality resistors are wound with only one layer of wire although this requires the use of smaller wire than for multilayer coils. The advantage of single layer coils are : 

(i) heat dissipation is more satisfactory with  single layer coils.On the other hand, in multi-layer coils, the central layers have a higher temperature. 

(ii)Single layer coils are more stable. Multi-layer are more sceptical to change in resistance t change in atmospheric humidity. 

(iii)Multi-layer coils are usually found to be less stable  in resistance with time.

Conclusion:
               Now here we have discussed the materials used in Resistors.If you want to go in depth for learning about resistor is made up of which material.

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