Pocketbook of Useful Formulæ and Memoranda
for Civil and Mechanical Engineers

306 Molesworth’s pocket-book Electbomagnets—continued. Let r = resistance of connections of battery system. 7 t — ab 9* U _ w 6 « (a + c) Z/2 ‘ F = JEJ = E a b__________________ R + H R g2 4- tt b a (a 4- c) ' _ E2 t2 Wa2b2 (R + H)2 [R g2 + tt b a (a + c)]2 ' Reduced value of R [relatively to H taken as resistance, in metres of helix] = . ju which. y-2 > case, as H will vary as gi instead of as g2, F - ^Ea6 9 R 9i + f2 TT b a (a + c) ’ A - g* E" a* & . [< Z R </4 + f2 tt b a (a 4- c)]’ ’ g ~ = diameter of bare wire. Maxima Conditions. As to resistance of helix:—For electromagnets of equal dimensions that size of wire is best which makes resistance of helix = R. But taking account of the thickness of insulating covering the resistance of the helix should be to R as diameter of bare wire is to that of same wire covered.