He also noted that pointed electrodes could be brought nearer together under oil than balls without allowing a discharge. Flat plates allowed of still greater sparking distances. Tesla notes that oil through which sparks have passed must be discarded, probably owing to particles of carbon being formed.
Paraffin wax has a higher resistance than oil, providing it has not been heated over 135° C. It will stand alternate heating up to 100° C. and cooling, being of lower resistance when hot than when cold. But a serious permanent deterioration takes place when it has been heated over 100°C.; its color, from the normal pure white, changes to a yellowish tint when its insulation is impaired. Paraffin also undergoes a deterioration when heated for a long time even at 100° C., and should never be used for fine work when it is at all yellow. It is always best to melt it in a hot-water bath, not permitting, however, any steam or moisture to come near it. In this climate (United States) it is not so necessary to mix in any tallow to obviate brittleness, the average temperature of most workshops being sufficiently high to keep it from becoming brittle.
Resin oils do not suffer permanent injury from being heated, as does paraffin, but their insulating properties diminish much more rapidly on becoming even warm, the initial resistance of resin oils being lower than that of paraffin.
Paraffin has a fault—its tendency to absorb a slight degree of moisture. It has been found in telephone and telegraph cables saturated with paraffin that this is a very important cause of their deterioration. In Ruhmkorff coils, however, which are intended for operation in enclosed places free from damp atmospheres, the absorption of moisture would be probably reduced to its minimum.
There is one substance which, were it not for its cost, would be far preferable to paraffin for coil work, and that is beeswax. Its cost, however, is generally five times that of paraffin, even when purchased in quantities. It never becomes brittle enough to be damaged in careful handling, its melting point is low, and it does not absorb moisture. But it must be unquestionably pure and clear.
In foreign practice a variety of resinous mixtures are used to insulate the turns of the wire in Ruhmkorff coils.
Equal parts of resin and beeswax used hot, paraffin, resin and tallow, and shellac and resin are employed.
Shellac—that is, the yellow lac—is much used as a varnish for electrical instruments, being dissolved in alcohol to saturation. For dynamo armatures and similar apparatus the shellac varnish is of great service, and many good compounds of shellac, such as insullac and armalac, have been prepared for ready use. But (excluding beeswax) for our purposes paraffin stands pre-eminently at the head of the list.
In using shellac varnish, in high tension work more particularly, care must be taken that the moisture has entirely evaporated. Although a piece of shellacked apparatus may appear perfectly dry, yet when the current is allowed to flow unlooked-for results may appear—it takes hours in a dry atmosphere for shellac varnish to dry. Baking the apparatus in a warm oven is a necessary expedient whenever feasible, care being taken not to burn or decompose the shellac. The proportions most generally used are 1 ounce shellac to 5 ounces alcohol. Stand the vessel containing the mixture in a warm place, and shake it frequently; filtration improves the varnish somewhat.
A ready and efficient varnish for silk is prepared by mixing 6 ounces of boiled linseed-oil and 2 ounces of rectified spirits of turpentine. For paper, 1 part of Canada balsam and 2 parts of spirits of turpentine dissolved in a warm place and filtered before being used. A good insulating cement for Leyden jars and insulating stands is prepared from sulphur, 100 parts; tallow, 2 parts, and resin, 2 parts, melted together until of the consistence of syrup, and sufficient powdered glass added to make a paste. To be heated when applied, this will resist most acids. The resin and beeswax compound is handy when making experimental mercurial air pumps of glass tubes, as it has a fair tenacity, is not too brittle, and is easily used.