Since Faraday’s time, by improved methods, but depending essentially upon the same principles, an enormous number of determinations of the dielectric constants of various insulators, solid, liquid and gaseous, have been made (see tables I., II., III. and IV.). There are very considerable differences between the values assigned by different observers, sometimes no doubt due to differences in method, but in most cases unquestionably depending on variations in the quality of the specimens examined. The value of the dielectric constant is greatly affected by the temperature and the frequency of the applied electric force.
Table II.—Dielectric Constant (K) of Liquids.
| Liquid. | K. | Authority. |
| Water at 17° C. | 80.88 | F. Heerwagen |
| ” ” 25° C. | 75.7 | E.B. Rosa |
| ” ” 25.3° C. | 78.87 | Franke |
| Olive oil | 3.16 | Hopkinson |
| Castor oil | 4.78 | ” |
| Turpentine | 2.15 | P.A. Silow |
| ” | 2.23 | Hopkinson |
| Petroleum | 2.072 | Silow |
| ” | 2.07 | Hopkinson |
| Ethyl alcohol at 25° C. | 25.7 | Rosa |
| Ethyl ether | 4.57 | Doule |
| ” ” | 4.8 | Bouty |
| Acetic acid | 9.7 | Franke |
Table III.—Dielectric Constant of some Bodies at a very low Temperature (−185° C.) (Fleming and Dewar).
| Substance. | K at 15° C. | K at −185°C. |
| Water | 80 | 2.4 to 2.9 |
| Formic acid | 62 | 2.41 |
| Glycerine | 56 | 3.2 |
| Methyl alcohol | 34 | 3.13 |
| Nitrobenzene | 32 | 2.6 |
| Ethyl alcohol | 25 | 3.1 |
| Acetone | 21.85 | 2.62 |
| Ethyl nitrate | 17.7 | 2.73 |
| Amyl alcohol | 16 | 2.14 |
| Aniline | 7.5 | 2.92 |
| Castor oil | 4.78 | 2.19 |
| Ethyl ether | 4.25 | 2.31 |
The above determinations at low temperature were made with either a steady or a slowly alternating electric force applied a hundred times a second. They show that the dielectric constant of a liquid generally undergoes great reduction in value when the liquid is frozen and reduced to a low temperature.[13]
The dielectric constants of gases have been determined by L. Boltzmann and I. Klemenčič as follows:—
Table IV.—Dielectric Constants (K) of Gases at 15° C. and 760 mm. Vacuum = 1.
| Gas. | Dielectric Constant K. | √K. | Optical Refractive Index. μ. |
| Air | 1.000590 | 1.000295 | 1.000293 |
| Hydrogen | 1.000264 | 1.000132 | 1.000139 |
| Carbon dioxide | 1.000946 | 1.000475 | 1.000454 |
| Carbon monoxide | 1.000690 | 1.000345 | 1.000335 |
| Nitrous oxide | 1.000994 | 1.000497 | 1.000516 |
| Ethylene | 1.001312 | 1.000656 | 1.000720 |
| Marsh gas (methane) | 1.000944 | 1.000478 | 1.000442 |
| Carbon bisulphide | 1.002900 | 1.001450 | 1.001478 |
| Sulphur dioxide | 1.00954 | 1.004770 | 1.000703 |
| Ether | 1.00744 | 1.003720 | 1.00154 |
| Ethyl chloride | 1.01552 | 1.007760 | 1.001174 |
| Ethyl bromide | 1.01546 | 1.007730 | 1.00122 |