Pocketbook of Useful Formulæ and Memoranda
for Civil and Mechanical Engineers

346 Molesworth’s pocket-book Electrical Formula, by Paget Higgs, LL.D., C.E.— continued. 44. Tension spark in air: Distance. Electrostatic Centimétre. Units = a. •0086 2'30 •0127 3’26 •0190 4-26 •0281 5’6-1 •0408 6-18 •0563 8-11 •0584 8-15 •0688 9-69 •0904 12-20 •1056 13-95 •1325 17 36 (Thomson). = Daniell’s elements. v -000374 45. Measurement of short intervals of time: By discharge of condenser : f = capacity of condenser in microfarads ; Il = its insulation resistance: P — potential before,/» = potential after time f (seconds), /Jog. €P t = -—P, R + r R r where r is resistance inserted between condenser- terminals in fraction of megohms. If R be infinite p to r, then t = rf log. € —. P By fall of charge on electrometer: P and p are the electrometer readings before and after the time t. By taking r small, very minute intervals of