LIGHTNING AND THUNDER. 219 



ORNAMENTAL IRON WORK. All vanes, finials, 

 ridge iron work, and similar ornamental metal work, 

 should be connected with the conductor : and it is not 

 absolutely necessary to use any other point than that 

 afforded by such ornamental work ; provided the con- 

 nection be perfect, and the mass of iron considerable. 

 As, however, there is risk of derangement through re- 

 pairs, it is safer to have an independent upper- terminal. 



MATERIAL FOR CONDUCTOR. The best material for 

 a conductor is copper ; its weight not less than six 

 ounces per foot run ; and its conductivity not less than 

 ninety per cent, of that of pure copper. It may be 

 used either in the form of tape, or of wire cable, in 

 which 110 wire should be less than No. 12 B. W. G. 

 Iron may be used, but its weight should not be less 

 than 2i poundsj^er foot run. And all iron conductors, 

 whether galvanized or not, should be painted, as a 

 protection against oxidation. Copper conductors may be 

 painted or not according to architectural requirements. 



FORM OF CONDUCTOR. The form of the conductor 

 does not seriously affect its conductivity : and great ex- 

 tent of surface in proportion to mass is not essential : but 

 sectional area of mass is highly essen tial,and should al ways 

 be sufficient to carry the heaviest charge without dan- 

 ger of fusion of the conductor, or division of the current. 



The rod is desirable for long upper terminals, on 

 account of its rigidity ; but the necessity of frequent 

 joints, and the difficulty of avoiding disfigurement of 

 the building, are serious objections to its use for the 

 body of the conductor. 



Tubes are liable to the same objections ; their larger 

 diameter, and the collars necessary for their joints, ren- 

 dering them more conspicuous and undesirable. 



