Pocketbook of Useful Formulæ and Memoranda
for Civil and Mechanical Engineers

307 OF ENGINEERING FORMTTLÆ. Electromagnets—continued. Between several coila wound with same wire but with different numbers of convolutions, for a circuit of given resistance, that will be best, the resistance of which is to B as a + c : a. As to a and c:—a should = c; b — 12. Where = TTi, 75’4 c3 12 c2 H then = -------— , and t = —— • R = II; a = c; b = m c; 2 ir c3m c3 2 7r m —7? - ------ä----’ and 9 ~ f ~ET-------• f2 g2 R q conductivity of iron wire n As —-J—-7-77—?------------r- = 6; and diam. conductivity of copper wire of 4 mm. wire = '000016 in.; q = ---------= ’ * -000016 2 7T m 375000, and -----= ’00020106 ; whence q •00020106. With relation to b:— E2 m? c4 os [R g2 + 2 ir c3 mJ2 ’ With relation to c :— With relation to P:— IM2 c< “ •ÖÖÜ08555' x 2