The resistivity of non-metallic conductors is in all cases higher than that of any pure metal. The resistivity of carbon, for instance, in the forms of charcoal or carbonized organic material and graphite, varies from 600 to 6000 microhms per cubic centimetre, as shown in Table VI.:—
Table VI.—Electric Volume-Resistivity in Microhms per Centimetre-cube of Various Forms of Carbon at 15° C.
| Substance. | Resistivity. |
| Arc lamp carbon rod | 8000 |
| Jablochkoff candle carbon | 4000 |
| Carré carbon | 3400 |
| Carbonized bamboo | 6000 |
| Carbonized parchmentized thread | 4000 to 5000 |
| Ordinary carbon filament from glow-lamp | |
| “treated” or flashed | 2400 to 2500 |
| Deposited or secondary carbon | 600 to 900 |
| Graphite | 400 to 500 |
The resistivity of liquids is, generally speaking, much higher than that of any metals, metallic alloys or non-metallic conductors. Thus fused lead chloride, one of the best conducting liquids, has a resistivity in its fused condition of 0.376 ohm per centimetre-cube, or 376,000 microhms per centimetre-cube, whereas that of metallic alloys only in few cases exceeds 100 microhms per centimetre-cube. The resistivity of solutions of metallic salts also varies very largely with the proportion of the diluent or solvent, and in some instances, as in the aqueous solutions of mineral acids; there is a maximum conductivity corresponding to a certain dilution. The resistivity of many liquids, such as alcohol, ether, benzene and pure water, is so high, in other words, their conductivity is so small, that they are practically insulators, and the resistivity can only be appropriately expressed in megohms per centimetre-cube.
In Table VII. are given the names of a few of these badly-conducting liquids, with the values of their volume-resistivity in megohms per centimetre-cube:—
Table VII.—Electric Volume-Resistivity of Various Badly-Conducting Liquids in Megohms per Centimetre-cube.
| Substance. | Resistivity in Megohms per c.c. | Observer. |
| Ethyl alcohol | 0.5 | Pfeiffer. |
| Ethyl ether | 1.175 to 3.760 | W. Kohlrausch. |
| Benzene | 4.700 | |
| Absolutely pure water approximates probably to | 25.0 at 18° C. | Value estimated by F. Kohlrausch |
| and A. Heydweiler. | ||
| All very dilute aqueous salt solutions having a | 1.00 at 18° C. | From results by F. Kohlrausch |
| concentration of about 0.00001 of an equivalent | and others. | |
| gramme molecule[10] per litre approximate to |
The resistivity of all those substances which are generally called dielectrics or insulators is also so high that it can only be appropriately expressed in millions of megohms per centimetre-cube, or in megohms per quadrant-cube, the quadrant being a cube the side of which is 109 cms. (see Table VIII.).
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 109 cms.
| Substance. | Resistivity. | Temperature Cent. | |
| Mega-megohms per c.c. | Megohms per Quadrant-cube. | ||
| 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° |