Fig. 86. Fig. 87.
One, in which paraffin appears as a cement, is an insulating stand made out of a bit of gla.s.s or ebonite tube cemented into an Erlenmeyer flask, having its neck protected from dust when out of use by a rubber washer, the other a "petticoat" insulator made by cementing a flint gla.s.s bottle into a gla.s.s dish with paraffin. In course of time the paraffin will be found to have separated from the gla.s.s. When this occurs the apparatus may be melted together again by placing it on the water bath for a few minutes.
-- 113. Vaseline, Vaseline Oil, and Kerosene Oil.
These, when dry, insulate almost, but not quite as well as solid paraffin. H. Koeller (Wien Berichte, 98, ii. 201, 1889; Beibl. Wied.
Ann. 1890, p. 186), working with very small voltages, places the final(?) specific resistance of commercial petroleum, ether, and vaseline oil at about 2 X 1027 C.G.S. This is ten times higher than the value a.s.signed to commercial benzene (C6H6), and a hundred times higher than the value for commercial toluene.
All these numbers mean little or nothing, but the petroleum and vaseline oil were the best fluid insulators examined by Koeller. By mixing vaseline with paraffin a soft wax may be made of any desired degree of softness, and by dissolving vaseline in kerosene an insulating liquid of any degree of viscidity may be obtained.
Hard paraffin may be softened somewhat by the addition of kerosene, and an apparently h.o.m.ogeneous ma.s.s cast from the mixture. It will be found, however, that in course of time the kerosene oozes out, unless present in very small quant.i.ty. 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 gla.s.s, 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 quant.i.tative 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
[Footnote: Professor J. J. Thomson, and Newall (Phil. Proc. 1886) consider that carbon bisulphide showed traces of a "residual charge"
effect; hence, until this point is cleared up, we must regard Bouty"s value as corresponding only to a very short, but not indefinitely short, period of charge. On this point the paper must be consulted.
March 1897--The writer has investigated this point by an independent method, but found no traces of "residual charge."]
Information as to the specific inductive capacity of a large number of oils may be found in a paper by Hopkinson, Phil. Proc. 1887, and in a paper by Quincke in Wiedemann"s Annalen, 1883.
-- 114. Imperfect Conductors.
Under this heading may be grouped such things as wood, slate, marble, etc--in fact, materials generally used for switchboard insulation.
An examination of the insulating power of these substances has recently been made by B. O. Peirce (Electrical Review, 11th January 1895) with quite sufficient accuracy, having in view the impossibility of being certain beforehand as to the character of any particular sample. The tests were made by means of holes drilled in slabs of the material to be examined. These holes were three-eighths of an inch in diameter, and from five-eighths to three-quarters of an inch deep, and one inch apart, centre to centre. A voltage of about 15 volts was employed. The following general results were arrived at:-
(1) Heating in a paraffin bath always increases the resistance of wood, though only slightly if the wood be hard and dense.
(2) Frequent exhaustion and readmission of air above the surface of the paraffin always has a good effect in increasing the resistance of wood.
(3) When wood is once dry, impregnating it with paraffin tends to keep it dry.
(4) Red vulcanised fibre, like wood, absorbs paraffin, but it cannot be entirely waterproofed in this way.
(5) The resistance of wood with stream lines along the grain is twenty to fifty per cent lower than when the stream lines cross the grain.
(6) The "contact" resistance between slabs of wood pressed together is always very high.
The following table will sufficiently ill.u.s.trate the results obtained.
The stone was dried in the sun for three weeks in the summer (United States), and the wood is described as having been well seasoned:-
CURRENT WITH THE GRAIN
Lowest Resistance Highest Resistance Lowest Specific Highest Specific between two Cups between two Cups Resistance in Resistance in in Megohms. in Megohms. Megohms. Megohms.
Ash.
550 920 380 700
Cherry
1100 4000 2800 6000
Mahogany