Pocketbook of Useful Formulæ and Memoranda
for Civil and Mechanical Engineers

340 Molesworth’s pocket-book Electrical Formulas, by Paget Higgs, LL.D., C.E., and R. S. Bkough—continued. Dimensions of e : m? l^ t * I e _ 1 __ „ E ; I E ü «: i s: F1t2 s 8 v = about 3 x 108 metres per second. 29. The range of any receiving instrument is — _ -P (g / + r + «) °" Q(.Pf+r) ' where r = resistance of instrument; w = resist- ance added; f = resistance of mean cell; p — number of cells employed to produce strong cur- rent O'; and q = number of cells employed to produce weak current C". 30. To correct wire resistance for temperature: 1 + (t1 - 32) a lw' ~ 1 +' (f _ 32) a Fahrenheit. lt(> = { 1 + (t' — t) a } Ef, Celsius. 31. 1 sq. in. = 645’148 sq, mm. log. 645-148 = 2-8096594. 1 sq. mm. = 0’00155 sq. in. log. 0-00155 = 3'1903317.