Pocketbook of Useful Formulæ and Memoranda
for Civil and Mechanical Engineers

332 Molesworth’s pocket-book Electrical Formula, by Paget Higgs, LL.D., C.E., and R. S, Brough—continued. where d Is the diameter of the wire, and h ita height above the ground. I S = --------r— microfarads per I miles. 4 h 24 log. — d 12. To measure the insulation of a cable by its loss of charge (Werner Siemens) : I = 0 ‘ 4343 -—-—-- megohms, Slog.- V where t is expressed in seconds and 8 in micro- farads. Res. per knot = / l°g-d \ 2-13 t ? ------- J (in any units), ' log.-/ V where V and v are potentials, and D and d diame- ters in any units. To measure insulation of a core, or cable, by direct deflection (Thomson galvanometer): R d \ I \ 8 / I \------— / I — insulation x al 7 in millions of units, where I length in knots, d = deflection with battery and 11 millions units