The temperature coefficients of pure metals are always positive and for some of the more familiar metals, have values, according to Foster, as in Table V.

Iron, it will be noticed, has the highest temperature coefficient of all. Carbon, on the other hand, has a large negative coefficient, as proved by the fact that the filament of an ordinary incandescent lamp has nearly twice the resistance when cold as when heated to full candle-power.

Certain alloys have been produced which have very low temperature coefficients, and these are of value in producing resistance units which have practically the same resistance for all ordinary temperatures. Some of these alloys also have very high resistance as compared with copper and are of value in enabling one to obtain a high resistance in small space.

One of the most valuable resistance wires is of an alloy known as German silver. The so-called eighteen per cent alloy has approximately 18.3 times the resistance of copper and a temperature coefficient of .00016 per degree Fahrenheit. The thirty per cent alloy has approximately 28 times the resistance of copper and a temperature coefficient of .00024 per degree Fahrenheit.

For facilitating the design of resistance coils of German silver wire, Tables VI and VII are given, containing information as to length, resistance, and weight of the eighteen per cent and the thirty per cent alloys, respectively, for all sizes of wire smaller than No. 20 B. & S. gauge.

Special resistance alloys may be obtained having temperature coefficients as low as .000003 per degree Fahrenheit. Other alloys of nickel and steel are adapted for use where the wire must carry heavy currents and be raised to comparatively high temperatures thereby; for such use non-corrosive properties are specially to be desired. Such wire may be obtained having a resistance of about fifty times that of copper.

TABLE VI

18 Per Cent German Silver Wire

No. B. & S.
Gauge		Diameter
Inches		Weight
Pounds per Foot		Length
Feet per Pound		Resistance
Ohms per Foot
21.02846.002389418.6.2333
22.02535.001894527.9.2941
23.02257.001502665.8.3710
24.02010.001191839.5.4678
25.01790.00094491058..5899
26.01594.00074931335..7438
27.01419.00059431683..9386
28.01264.00047112123.1.183
29.01126.00037352677.1.491
30.01003.00029623376.1.879
31.008928.00023504255.2.371
32.007950.00018645366.2.990
33.007080.00014786766.3.771
34.006304.00011728532.4.756
35.005614.0000929510758.5.997
36.005000.0000736913569.7.560
37.004453.0000584517108.9.532
38.003965.0000463621569.12.02
39.003531.0000367527209.15.16
40.003145.0000291734282.19.11