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Chapter: 2 General physics
    Section: 2.6 Electricity and magnetism
        SubSection: 2.6.2 Resistance alloys and wire resistances

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2.6.2 Resistance alloys and wire resistances

Copper–manganese alloys (~84% Cu, 12% Mn with nickel, aluminium or germanium as the remaining constituent). These alloys are sold under various proprietary names, and manganin, the pioneer alloy of this group, was for many years the traditional material for high-grade standard resistors. The resistivity is about 40 × 10−8 Ω m and varies approximately parabolically with temperature over the range 0 to 50 °C, with a maximum close to 20 °C. The temperature coefficient can be as low as 3 × 10−6 °C−1 over the range 15 °C to 20 °C. Its secular stability is very good and, if wires are supported in strain-free conditions, can be less than 1 in 107 per year. The thermo-e.m.f. of the alloys against copper is close to zero and may be positive or negative according to composition and heat treatment. Joints between the copper manganese alloys and copper are made most effectively by welding in an atmosphere of argon, and by hard soldering if welding is impracticable.

Copper–nickel alloys (~55% Cu, 45% Ni). These alloys are manufactured commercially under a wide range of proprietary names, and are used in the construction of standard resistors. The resistivity is about 50 × 10−8 Ω m with a temperature coefficient which may lie between ±0.000 04 °C−1.
  The alloys can be soft-soldered with ease, but their high thermo-e.m.f. against copper (~40 μV °C−1) is a disadvantage in d.c. resistors, although the effect is usually negligible in a.c. resistors dropping 1 volt or more. These alloys are also used for current controlling resistors when constancy is more important than low cost.

The table gives resistance of wires in ohms per metre and approximate currents in amperes required to maintain stated temperature rises in straight horizontal wires of nickel–chromium free to radiate in air.


Properties of resistance wires

S.W.G

Diameter

Copper

Copper–
Manganese
alloys

Copper–
nickel
alloys

Quaternary
alloys

Nickel–chromium

mm

in

Ω m−1
at 20°C

Ω m−1

Ω m−1

Ω m−1

Ω m−1

Current (A) to
maintain temp. rise

500 °C

1000 °C

 

 

 

 

 

 

 

 

 

 

12

2.642

0.104  

     0.003 12

  0.076

0.090

        0.243

     0.197

38    

78   

14

2.032

0.080  

     0.005 32

  0.128

0.151

        0.410

     0.333

26    

53   

16

1.626

0.064  

     0.008 31

  0.200

0.235

        0.640

     0.520

19     

40   

 

 

 

 

 

 

 

 

 

 

18

1.219

0.048  

   0.014 8

  0.355

0.420

      1.14

   0.92

13     

27    

20

0.914

0.036  

   0.026 3

  0.630

0.745

       2.03

   1.65

8.5

18    

22

0.711

0.028  

   0.043 4

1.05

1.23  

        3.35 

   2.72

6.3

13    

 

 

 

 

 

 

 

 

 

 

24

0.559

0.022  

   0.070 3

1.69

2.00  

         5.40  

   4.40

4.5

9.5

26

0.457

0.018  

0.105

2.53

3.00  

         8.1    

   6.60

3.5

7.0

28

0.376

0.0148

0.155

3.75

4.40  

     12.0  

  9.7

2.7

5.5

 

 

 

 

 

 

 

 

 

 

30

0.315

0.0124

0.221

5.30

6.30  

     17.1  

13.9

2.2

4.5

32

0.274

0.0108

0.292

7.00

8.30  

     22.5  

18.3

1.9

3.5

34

0.234

0.0092

0.402

9.7  

11.4     

     31.0  

25.2

1.6

3.0

 

 

 

 

 

 

 

 

 

 

36

0.193

0.0076

0.589

14.2    

16.7     

     45.5  

37.0

1.3

2.3

38

0.152

0.0060

0.945

22.7    

27.0     

     73.0  

59.0

1.0

1.7

40

0.122

0.0048

1.48  

35.5    

42.0    

114  

92   

0.8

1.4

 

 

 

 

 

 

 

 

 

 

42

0.102

0.0040

2.13  

51.0    

60.5    

164  

133     

  0.65

1.1

44

  0.0813

0.0032

3.32  

80       

94       

255  

208    

46

  0.0610

0.0024

5.91  

142         

168         

455  

370    

 

 

 

 

 

 

 

 

 

 

48

  0.0406

0.0016

13.3     

320         

380        

1030    

835    

50

  0.0254

0.0010

34.0     

820         

970       

2620    

2130     

 

 

 

 

 

 

 

 

 

 


Nickel–chromium alloys (~80% Ni, 20% Cr). These alloys are also available under a variety of trade names and are used for heater elements as well as for resistors where high accuracy is not required. Their resistivity is about 110 × 10−8 Ω m with a temperature coefficient (20–500 °C) of 0.000 06 °C−1. They will operate satisfactorily at temperatures of up to 1100 °C. A ternary alloy (65% Ni, 15% Cr, 20% Fe) is less expensive than nickel–chromium and is satisfactory at temperatures up to 900 °C.

A particular development of 80/20 nickel–chromium for reference or standard resistors is the use of thick-film deposits on glass substrates. These resistors are essentially thermally-compensated strain gauges in which the change of resistance with temperature counteracts the change due to the strain imparted by the substrate. When suitably heat-treated after deposition, such resistors can achieve temperature coefficients of not more than a few p.p.m. and secular stability comparable with that of manganin or quaternary alloys. However, because of their very small size, they can be used at much higher frequencies than is possible with wire-wound types.

Quaternary alloys (~73% Ni, 21% Cr, 2% Al, with copper, iron, cobalt, manganese or molybdenum as the fourth main constituent). These alloys are again processed and marketed under many proprietary names, each of which is characterized by the fourth constituent; they are increasingly being used for standard resistors, especially those of high value. Their resistivity is about 130 × 10−8 Ω m, with a temperature coefficient controllable by heat treatment and which can be made as small as ±0.000 002 °C−1. The thermo-e.m.f. of the alloys, like that of the copper–manganese alloys, is close to zero and may be slightly positive or negative. The tensile strength of the alloys is high, making possible the drawing of very fine wire. Welding in argon is by far the best method of joining with copper, with hard soldering as an alternative only if welding is not possible.

C.H.Dix

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