TABLE VIII.--_Electric Volume-Resistivity of Dielectrics reckoned in Millions of Megohms (Mega-megohms) per Centimetre-cube, and in Megohms per Quadrant-cube, i.e. a Cube whose Side is 10^9 cms._ +------------------------+---------------------------+----------+ | | Resistivity. | | | +------------+--------------+ Tempera- | | Substance. |Mega-megohms| Megohms per | tura | | | per c.c. |Quadrant-cube.| Cent. | +------------------------+------------+--------------+----------+ |Bohemian glass | 61 | .061 | 60° | |Mica | 84 | .084 | 20° | |Gutta-percha | 450 | .45 | 24° | |Flint glass | 1,020 | 1.02 | 60° | |Glover's vulcanized | | | | | indiarubber | 1,630 | 1.63 | 15° | |Siemens' ordinary pure | | | | | vulcanized indiarubber | 2,280 | 2.28 | 15° | |Shellac | 9,000 | 9.0 | 28° | |Indiarubber | 10,900 | 10.9 | 24° | |Siemens' high-insulating| | | | | fibrous material | 11,900 | 11.9 | 15° | |Siemens' special | | | | | high-insulating | | | | | indiarubber. | 16,170 | 16.17 | 15° | |Flint glass | 20,000 | 20.0 | 20° | |Ebonite | 28,000 | 28. | 46° | |Paraffin | 34,000 | 34. | 46° | +------------------------+------------+--------------+----------+ 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° C., then we can write [rho]t = [rho]0(1 + [alpha]t), or [alpha] = ([rho]t - [rho]0)/[rho]0t.