Pocketbook of Useful Formulæ and Memoranda
for Civil and Mechanical Engineers

331 OF ENGINEERING FOBMULÆ. Electrical Formula, by Paget Higgs, LTj.D., C.E,, and R. S. BeoüGH—continued. broad rule g.p. having high insulating properties, has also high inductive capacity. 10. The electrostatic capacity of any cable is c 1 s —-----------electrostatic units, . D 2 log. e - where c = 1 for air, =3'1 for Hooper’s core, = 4 • 2 for guttapercha. s I S = 2*7------p microfarads per I knots, 'og.J where s is the inductive capacity of a cubic knot of the dielectric. s = 0-0687 microfarad for guttapercha. s = 0'0543 microfarad for Hooper’s core. The electrostatic capacity in microfarads of a guttapercha cable is approximately per knot k ~ log. D - log. d ’ where k varies from -1800 to -1400, the lower number corresponding to the lower figure for in- sulation given in (9), and vice versa. 11. The electrostatic capacity of an overland line is . g -----------— electrostatic units, 4/j