per | tura |
| | per c.c. |Quadrant-cube.| Cent. |
+------------------------+------------+--------------+----------+
|Bohemian glass | 61 | .061 | 60 deg. |
|Mica | 84 | .084 | 20 deg. |
|Gutta-percha | 450 | .45 | 24 deg. |
|Flint glass | 1,020 | 1.02 | 60 deg. |
|Glover's vulcanized | | | |
| indiarubber | 1,630 | 1.63 | 15 deg. |
|Siemens' ordinary pure | | | |
| vulcanized indiarubber | 2,280 | 2.28 | 15 deg. |
|Shellac | 9,000 | 9.0 | 28 deg. |
|Indiarubber | 10,900 | 10.9 | 24 deg. |
|Siemens' high-insulating| | | |
| fibrous material | 11,900 | 11.9 | 15 deg. |
|Siemens' special | | | |
| high-insulating | | | |
| indiarubber. | 16,170 | 16.17 | 15 deg. |
|Flint glass | 20,000 | 20.0 | 20 deg. |
|Ebonite | 28,000 | 28. | 46 deg. |
|Paraffin | 34,000 | 34. | 46 deg. |
+------------------------+------------+--------------+----------+

A definition may here be given of the meaning of the term _Temperature
Coefficient_. If, in the first place, we suppose that the resistivity
([rho]t) at any temperature (t) is a simple linear function of the
resistivity ([rho]0) at 0 deg. C., then we can write [rho]t = [rho]0(1
+ [alpha]t), or [alpha] = ([rho]t - [rho]0)/[rho]0t.

The quantity [alpha] is then called the temperature-coefficient, and
its reciprocal is the temperature at which the resistivity would
become zero. By an extension of this notion we can call the quantity
d[rho]/[rho]dt the temperature coefficient corresponding to any
temperature t at which the resistivity is [rho]. In all cases the
relation between the resistivity of a substance and the temperature is
best set out in the form of a curve called a temperature-resistance
curve. If a series of such curves are drawn for various pure metals,
temperature being taken as abscissa and resistance as ordinate, and if
the temperature range extends from the a