The silver is first melted in a plumbago crucible in a small furnace together with a little borax; if any copper is required this is then added, and finally the brass is introduced. When fusion is complete, the contents of the crucible are poured into any suitable mould.
The quickest and most convenient way of preparing the alloy for use is to convert it into filings with the assistance of a coarse file, or by milling it, if a milling machine is available.
Equal volumes of filings and powdered glass borax are made into a thin paste with water, and applied in an exactly similar manner to that described under the head of "brazing." In fact all the processes there described may be applied equally to the case under discussion, the substitution of silver for spelter being the only variation.
The silver solder is more manageable than spelter, and does not tend to run wild over the work: a property which makes it much more convenient both for delicate joints and in cases where it is desired to restrict the solder to a single point or line. Small objects are almost invariably soldered with silver solder, and are held by forceps or on charcoal in the pointed flame of an ordinary blow-pipe.
[§ 101. On the Construction of Electrical Apparatus - Insulators. —]
It is not intended to deal in any way with the design of special examples of electrical apparatus, but merely to describe a rather miscellaneous set of materials and processes constantly required in its construction.
It is not known whether there is such a thing as a perfect insulator, even if we presuppose ideal circumstances. Materials as they exist must be regarded merely as of high specific resistance, that is if we allow ourselves to use such a term in connection with substances, conduction through which is neither independent of electromotive force per unit length, nor of previous history.
Even the best of these substances generally get coated with a layer of moisture when exposed to the air, and this as a rule conducts fairly well. Very pure crystalline sulphur and fused quartz suffer from this defect less than any other substances with which the writer is acquainted, but even with them the surface conductivity soon grows to such an extent as totally to mask the internal conduction.
It is proposed to give a brief account of the properties of some insulating substances and their application in electrical construction, and at the same time to indicate the appliances and methods requisite for working them.
With regard to the specific resistances which will be quoted, the numbers must not be taken to mean too much, partly for the reason already given. It is also in general doubtful whether sufficient care has been taken to distinguish the body from the surface conductivity, and consequently numerical estimates are to be regarded with suspicion. The question of "sampling" also arises, for it must be remembered that a change in composition amounting to, say, 1/10000 per cent may be accompanied by a million-fold change in specific resistance.