Pocketbook of Useful Formulæ and Memoranda
for Civil and Mechanical Engineers

330 Molesworth’s pocket-book Electrical Formula, by Paget Higgs, LL.D., C.E., and K. S. Brough—continued. (6) By the capacity method : where S = total capacity when fault free, and s = measured capacity, L being length of line. 8. When a line contains several different gauges of wire, to find the resistance per mile of any particular gauge (Schwendler) : ' L % a — T , where da = the diameter of the wire whose re- sistance is required. 9. The insulation of any cable is T Ä 1 D K I = —log. - megohms i 7T( Cl — 0 • 3665 -J log. ~ megohms, where k is the resistance in megohms of a cubic unit of the insulating material. When the knot = 2029 yards is the unit of length, k = 2100 megohms at 75° F. for gutta- percha, k = 40950 megohms at 753 F. for Hooper’s core. The insulation of any guttapercha cable is p (log. D — log. d) megohms per knot at 75° F., ■where p varies from 700 to 300, according to the insulating property of the g.p. coating. As a