Pocketbook of Useful Formulæ and Memoranda
for Civil and Mechanical Engineers

322 Molesworth’s pocket-book Electrical Formula, by Paget Higgs, LL.D., C.E., and R. S. Brough—continued. ’ 2. The measurement of resistance. In the following let a and b be resistances in ratio-arms of bridge, io balancing, ami x unknown resistance. 1. By Wheatstone’s Bridge: b x ~ - . w. a When measures with ± currents differ slightly, and w' is reading with + current, and io" read- ing with — current towards a and b, b io" + w’ x = - .---------------.. a 2 If the difference is greater, & (w" + w' w" — w) «12 N 2 / ’ let E = e. m. f. of battery, and e that in one of tne sides, e = NE, where , >\ y ______________b(w — to) ___________ b (w" + w) + 2 { ab+f (a + b) } * Neglecting/, the battery resistance, XT W” ~ IO' N = —-------------- w + ?,p' + a 11. By the differential galvanometer: n % ~ . w. h