Hard paraffin may be softened somewhat by the addition of kerosene, and an apparently homogeneous mass cast from the mixture. It will be found, however, that in course of time the kerosene oozes out, unless present in very small quantity. Koeller has found (loc. cit.) that some samples of vaseline oil conducted "vollstaendig gut," but I have not come across such samples. Vaseline oil, however, is sold at a price much above its value for insulating purposes.
Kerosene oil is best obtained dry by drawing it directly from a new tin and exposing it to air as little as possible. Of course, it may be dried by chemical means and distillation, but this is usually (or always) unnecessary.
Fig
88.
There is some danger of kerosene containing minute traces of sulphuric acid, and it and other oils may be conveniently tested for insulation in the following manner. The quartz electroscope is taken, and the insulating rod heated in the blow-pipe. The electroscope will now insulate well enough to show no appreciable collapse of the leaves in one or two hours' time. Upon the plate of the electroscope is put a platinum or copper cylinder, and this is filled with kerosene (say) up to a fixed mark.
The electroscope is placed on a surface plate, or, at all events, on a sheet of plate glass, and a "scribing block" is placed along side it and the scriber adjusted to dip into the kerosene to any required depth. This is done by twisting a bit of wire round the scribing point and allowing it to project downwards. The point itself serves to give an idea of the height to which the vessel may be filled. The liquid is adjusted till its surface is in contact with the end of the scribing point, and the wire then projects into the liquid and forms an electrode of constant area of surface. The scribing block is put to earth. A charge is given to the electroscope, and the time required for a given degree of collapse of the leaves noted.
The kerosene is then removed and its place taken by vaseline or paraffin, known to insulate well as a standard for comparison. The experiment is then repeated, and the time noted for the same degree of collapse. This test, though of course rough, is generally quite sufficient for workshop purposes, and is easily applied. Moreover, it does not require correction for electrometer leakage, as generally happens when more elaborate appliances are used.
The actual resistance of insulating oils depends so much on the electrical intensity, on the duration of that intensity, and on the previous history of the oil as to the direction of the voltage to which it has been subjected — to say nothing of the effects of traces of moisture — that quantitative experiments are of no value unless they are extremely elaborate. I shall therefore only append the following numbers due to Bouty, Ann. de Chemie et de Physique (6), vol. xxvii. p. 62, 1892, in which the effect of the conductivity on the determination of the specific inductive capacity was properly allowed for:—
|
Carbon Bisulphide. | Turpentine. | Benzene (C6H6) at 20° C. |
Benzene at 60° C. | |
| Specific inductive capacity | 2.715 | 2.314 | 2.21 | 2.22 |
| [Specific resistance in ohms per cubic centimetre] | 1.5 x 1013, | 1.75 x 1012 | 1.56 x 1011 | 7.9 x 1011 |