Electrical Resistance Materials

A resistor is a device, which is used to introduce resistance in an electric circuit for the various purposes such as

(i)    for controlling µo current such as rheostats,
(ii)    as heater elements for converting electrical energy into heat energy and
(iii)    as meter elements such as ammeter shunts and voltmeter multipliers.

The principle requirements of materials employed for resistance, when used as rheostats are low cost, ling life, low temperature coefficient, high resistively and low thermo e.m.f. to copper. When used as heater elements there is an additional requirement that it should operate at high temperature and this usually conducts with the requirement of long life. When constant and permanent as possible. In particulars it should be independent of temperature over its range of operating temperature.

Alloys of nickel, chromium, copper, iron, manganese are extensively used as resistance materials. These alloys have high specific resistance (about 25 times that of pure copper) and very low temperature coefficient of resistance (about one twentieth of that of pure copper). In general the higher the specific resistance of the lateral, the shorter and stouter the resistance element and simple and stronger consecution. The various materials used as resistance materials are described below:

1.    Platinum. Though pure platinum is very good metal form the point of view of permanence but its use is limited due to its high cost, high temperature co-efficient of resistance (0 004 Ω/Ω/per⁰C). Its resistively is 0.47 µΩ-m. It is used in platinum resistance thermometer for measuring high temperatures, and in other scientific instruments such as thermo-couples.

2.    Tungsten. Tungsten is a silver-white hard metal obtained by a suitable chemical process form ore concentrates. It has the highest melting point of all metals (3.300⁰C). Ductile tungsten is used for lamp filaments, value electrodes, cathode-ray anticathode etc. It electrical properties are highly improved by alloying it with other materials. The specific resistance and temperature co-efficient of resistance of tungsten are 5.48 x 10⁸ Ω-m and 0.0045 ohm per ohm per ⁰C respectively.

3.    Manganin. It is an alloy of 86 per cent copper, 12 per cent manganese and percent nickel of a reddish brown colour. Its properties are :

Specific resistance 0.42-0.43 µΩ-m
Temperature coefficient of resistance 0.00001 to 0.00015 /Ω/⁰C
Thermo-e.m.f against copper about 5 µV⁰ / C
Density                                         8,400 Kg/m³
Tensile strength                             40-55 kg/mm²
Maximum working temperature Not over 60⁰C

It is most widely used material for the manufacture of resistance boxes, standard resistance and shunts.

4.    Constantan.  Constantan is an alloy of 60 percent copper and 40 percent nickel. Its properties are :
Resistivity                                       50 x 10^-8 Ω-m
Temperature coefficient of resistance   very small :may be either positive or negative.

Thermo-e.m.f. against copper                about 40 µ V/ ⁰ C
Density                                                8,900 kg/m³
Tensile strength                                    40-50 kg/mm²
Maximum working temp.                       400 to 450⁰C

It is widely used in the manufacture of theostas and heater resistors, provided their working temperature does not exceed 400 to 500⁰C. Due to developing of high thermo-e.m.f. when in a junction with copper it cannot be used in the manufacture of standard resistances for high precision instruments. The use of their property of constantan is made in thermo-couples for measuring temperature upto several hundred degrees.

5.    Nichrome. Nichrome is an alloy of nickel and chromium. The properties of nichrome are :

Resistivity                                           about 1.1 µΩ-m
Temperature coefficient of resistance     about 0.0002 Ω/Ω/⁰C
Density                                                8,400 Kg/m³
Tensile strength                                    70 kg/mm²
Working temperature                            upto 1,000⁰C

Nichrome is available in the form of a wire or tape and is widely used as heating element in domestic appliance (such as heater, electric iron, toaster etc.) and furnaces with working temperature upto 1000⁰C.

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