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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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