Patent GB-256458-A (GB)
Title: Improvements in or relating to the reduction of losses in transmission systems
IPC: H01B17/20
Filed: 1925-12-21
Abstract: 256,458. Standard Telephones & Cables, Ltd., (Western Electric Co., Inc.). Dec. 21, 1925. Insulators.-In the methods of transmitting telephonic and telegraphic signals by means of carrier currents propagated along open wire lines, the leakage losses through the insulators are eliminated by the provision of a path 10 of practically zero resistance between the dielectric of one insulator 6 carrying one of the conductors 1, and the dielectric of the insulator 7 of the other conducrtor 2. The insulators 6, 7 are mounted upon insulating or partially conducting cross arms 3, and metal pins 11, 12 or wooden pins sheathed with metal. The materials of which the insulators are constructed are selected to provide a low dielectric hysteresis loss, and the forms of the insulators are such as to reduce leakage loss in wet weather to a minimum, and to ensure that the capacity effect remains substantially constant in all weathers. The dielectric loss in the insulator is equivalent to a resistance R, Fig. 8, shunted across a condenser C, and the currents I<1>, I, Fig. 9, passing through the resistance and the condenser respectively, when plotted as a vector diagram, gives an angle # as a measure of the loss. The insulators are made of a material having a loss angle # of less than 20 minutes for the frequencies employed. As examples of such material glass having a large content of heavy metal, such as lead, antimony or barium is mentioned. One example has the following composition, viz. :-SiO2, 64.64 per cent; PbO, 21.66 per cent; Na2O, 9.10 per cent; R2O, 3.20 per cent; B2O3, 0.37 per cent; and small quantities of P2O5' Al2O3' Fe2O3' CaO, MgO, and fluorine, giving a loss angle of 12 minutes at a frequency of 30,000 cycles. A glass approximating to a boro-silicate glass, such as the glass known as Electrical Pyrex, is also used. Various forms of insulators made of these materials are described, their form and proportions being arranged to reduce the leakage losses. The insulator shown in Fig. 10 is screwed on to a wooden pin having a metal sheath, and the insulator has both an exterior flange 15 and an interior flange 16. In that of Fig. 11, the interior 19 is corrugated and adapted for a metal pin of smaller diameter. The capacity of the insulator is reduced by arranging the dimension R1 Fig. 14 to be four times that of the diameter R2 of the metal pin; the thickness of the material above the top of the pin being the same as that round the pin. Further modifications consist in providing the insulator with an outer conducting covering of metal, as shown in Fig. 17, which may be pressed against the lower part of the insulator or may stand away from it. In some cases the metal covering is not continued over the top of the insulator so that light may penetrate to the inner surface 25, and prevent bugs and other insects collecting in the cavity. The insulators may also be fitted with umbrella-like covers, Figs. 23 and 29, having a flaring rim extending outwardly to protect the outer surface in the neighbourhood of the tie wire from moisture. The under surface of the flaring rim may be corrugated.