Pocketbook of Useful Formulæ and Memoranda
for Civil and Mechanical Engineers

338 moleswokth’s pocket-book Electrical Formulae, by Paget Higgs, LL.D., C.E., and R. S. Brough—continued. of wire in inches; a = constant depending on method of coiling, generaily = 4; y = (constant) = • 0000001; d = diam. required in inches: ,, „ ylm • 0000002 Im 2 r2 (r + s) 2C a r2 (r 4- s) 2c ’ ’ Q +s) "2Gc’ ~V 2G7. + T~2 and as s is generally very small, y I m 2G~c s _ — and d = *7 ylm V 2Gc — s. 26 To test a telegraph earth (Schwendler): Take two auxiliary earths, and measure the resistance between each of thé three pairs of earths. (The mean of positive and negative readings to be taken.) Let x be the required resistance, and «, b, and c the three observed resistances, then a 4- b — c 1 By the tangent galvanometer: a? = ^±-?.tana°(< ^_+_1---------I--------- 2 \tanc° tand0 tan 6° tan ePJ where / and g are respectively = the resistances of the testing battery and galvanometer: and the deflections obtained are: with no external re-