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    |
+---------------------------+--------------+--------------+<

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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.