Temperature Coefficient

The design or selection of resistance devices for various purposes frequently involves the consideration of the effect of temperature on the resistance of the conductor employed. The resistance of conductors is subject to change by changes in temperature. While nearly all metals show an increase, carbon shows a decrease in its resistance when heated. The temperature coefficient of a conductor is a factor by which the resistance of the conductor at a given temperature must be multiplied in order to determine the change in resistance of that conductor brought about by a rise in temperature of one degree. TABLE V Temperature Coefficients | Pure Metals | Temperature Coefficients | | Centigrade | Fahrenheit | | Silver (annealed) | 0.00400 | 0.00222 | | Copper (annealed) | 0.00428 | 0.00242 | | Gold (99.9%) | 0.00377 | 0.00210 | | Aluminum (99%) | 0.00423 | 0.00235 | | Zinc | 0.00406 | 0.00226 | | Platinum (annealed) | 0.00247 | 0.00137 | | Iron | 0.00625 | 0.00347 | | Nickel | 0.0062 | 0.00345 | | Tin | 0.00440 | 0.00245 | | Lead | 0.00411 | 0.00228 | | Antimony | 0.00389 | 0.00216 | | Mercury | 0.00072 | 0.00044 | | Bismuth | 0.00354 | 0.00197 | Positive and Negative Coefficients. Those conductors, in which a rise in temperature produces an increase in resistance, are said to have positive temperature coefficients, while those in which a rise in temperature produces a lowering of resistance are said to have negative temperature coefficients. 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 |.002389 | 418.6 |.2333 | | 22 |.02535 |.001894 | 527.9 |.2941 | | 23 |.02257 |.001502 | 665.8 |.3710 | | 24 |.02010 |.001191 | 839.5 |.4678 | | 25 |.01790 |.0009449 | 1058. |.5899 | | 26 |.01594 |.0007493 | 1335. |.7438 | | 27 |.01419 |.0005943 | 1683. |.9386 | | 28 |.01264 |.0004711 | 2123. | 1.183 | | 29 |.01126 |.0003735 | 2677. | 1.491 | | 30 |.01003 |.0002962 | 3376. | 1.879 | | 31 |.008928 |.0002350 | 4255. | 2.371 | | 32 |.007950 |.0001864 | 5366. | 2.990 | | 33 |.007080 |.0001478 | 6766. | 3.771 | | 34 |.006304 |.0001172 | 8532. | 4.756 | | 35 |.005614 |.00009295 | 10758. | 5.997 | | 36 |.005000 |.00007369 | 13569. | 7.560 | | 37 |.004453 |.00005845 | 17108. | 9.532 | | 38 |.003965 |.00004636 | 21569. | 12.02 | | 39 |.003531 |.00003675 | 27209. | 15.16 | | 40 |.003145 |.00002917 | 34282. | 19.11 | TABLE VII 30 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 |.002405 | 415.8 |.3581 | | 22 |.02535 |.001907 | 524.4 |.4513 | | 23 |.02257 |.001512 | 661.3 |.5693 | | 24 |.02010 |.001199 | 833.9 |.7178 | | 25 |.01790 |.0009513 | 1051. |.9051 | | 26 |.01594 |.0007544 | 1326. | 1.141 | | 27 |.01419 |.0005983 | 1671. | 1.440 | | 28 |.01264 |.0004743 | 2108. | 1.815 | | 29 |.01126 |.0003761 | 2659. | 2.287 | | 30 |.01003 |.0002982 | 3353. | 2.883 | | 31 |.008928 |.0002366 | 4227. | 3.638 | | 32 |.007950 |.0001876 | 5330. | 4.588 | | 33 |.007080 |.0001488 | 6721. | 5.786 | | 34 |.006304 |.0001180 | 8475. | 7.297 | | 35 |.005614 |.00009358 | 10686. | 9.201 | | 36 |.005000 |.00007419 | 13478. | 11.60 | | 37 |.004453 |.00005885 | 16994. | 14.63 | | 38 |.003965 |.00004668 | 21424. | 18.45 | | 39 |.003531 |.00003700 | 27026. | 23.26 | | 40 |.003145 |.00002937 | 34053. | 29.32 |