On Lightning Conductors. 113 



directions given at pp. 12 to 14.* He must, however, bear 

 in mind that even ordinary chimney stacks, when exposed, 

 should be protected by short terminals connected to the 

 nearest rod, inasmuch as accidents often occur owing to the 

 good conducting power of the heated air and soot in a 

 chimney. 



3. Insulators. — The rod is not to be kept from the 

 building by glass or other insulators, but attached to it by 

 metal fastenings. (See p. llf). 



4. Fixing. — Rods should preferentially be taken down the 

 side of the building which is most exposed to rain. They 

 should be held firmly, but the holdfasts should not be driven 

 in so tightly as to pinch the rod, or prevent the contraction 

 and expansion produced by changes of temperature. 



5. Factory Chimneys. — These should have a copper 

 band round the top, and stout sharp copper points, each 

 about 1 foot long, at intervals of 2 or 3 feet through- 

 out the circumference, and the rod should be connected 

 with all bands and metallic masses in or near the chimney. 

 Oxidation of the points must be carefully guarded 

 against. 



6. Ornamental Ironwork. — All vanes, finials, ridge 

 ironwork, &c., sliould be connected with the conductor, and 

 it is not absolutely necessary to use any other point than 

 that afforded by such ornamental ironwork, provided the 

 connection be perfect, and the mass of ironwork considerable. 

 As, however, there is risk of derangement through repairs, 

 it is safer to have an independent upper terminal. 



* Considerable difference of opinion has existed as to the area protected by 

 each conductor. The latest French instructions, quoted on p. 13 of the 

 Report accompanying the Code of Rules, say a point will " effectively protect 

 a cone having the point for its apex and a base whose radius is 1*75 of its 

 height." The English War Department instructions say that " no precise 

 limit can be fixed to the protecting power of conductors. In England the 

 base of the protected cone is usually assumed to have a radius equal to the 

 height from the ground ; but, though this may be sufficiently correct for 

 practical purposes, it cannot always be relied upon." 



t Page 11 of Report: "The evidence against the use of glass or other 

 material in order to insulate the conductor is overwhelming, and insulation 

 may be regarded as unnecessary and mischievous. The essentials are — (1) 

 That the rod be attached to the building by fastenings of the same metal as 

 itself ; (2) that the fastenings be of adequate strength ; (3) that they be of 

 such a form as not to compress or distort the rod ; (4) that they allow for 

 expansion and contraction ; (5) that they hold it firmly enough to prevent 

 all the weight falhng on any one bearing." 



I 



