{Contributed by H. Y. Margary)

The term cable strictly applies to a conductor composed of a number of strands of wire. Cables are often spoken of as wires, conductors, or leads. The term wire, however, more strictly applies to a conductor composed of one strand only. The cables used for wiring a building for electric light are composed of drawn copper wires covered with some insulating material. When single wires are used they should not be smaller than No. 18 S.W.G. (Standard Wire Gauge) or greater than No. 16. The reason for limiting the size of single wires is that if too small they are liable to break, and if too large they are so inflexible that they cannot be easily drawn into casing or conduits, or adapted to awkward corners, without causing mechanical injury to the insulation. The smallest stranded cable in general use is composed of 3 strands of No. 22 S.W.G. A cable composed of 3 strands of No. 22 is written thus - 3/22, a cable composed of 19 strands of No. 20 is written 19/20, and so on. The sizes of cable in general use are given in the first column of the table on page 180.

Conductivity Of Cables

The copper used for the cables should be of at least 98 per cent. conductivity; that is to say, its conductivity should be at least 98 per cent, of the conductivity of chemically pure copper.

Insulation Of Cables

The copper of which the conductors are composed is covered with a coating of nonconducting material, or dielectric as it is called, to prevent the cables from touching and the current from escaping to earth. There are numerous materials used for insulating cables, such as indiarubber, guttapercha, paper, bitumenised fibre, etc. There are generally two descriptions of cable used for house wiring, - those insulated with a material as a dielectric, which is itself so impervious to moisture that it only needs further protection from mechanical injury and from vermin; and those insulated with a material as dielectric, which, in order to preserve its insulation qualities, must be kept perfectly dry. Rubber and gutta-percha are used in the former description of cable, in which case the dielectric is drawn immediately over the copper and protected from abrasion by a protective binding of cotton, silk, braid, or tape. Paper is used in the latter description of cable, and it is usually protected by an envelope of lead drawn closely over the dielectric, or else encased in thoroughly waterproof tubes.