Factor of Safety in the Materials for Lightning-rods. 171 

 given, we have 



Factor of safety for equal total cost = — ~ — =- 



p X Uj X K 



The only metals about which it is necessary to inquire are 

 copper, silver, iron, lead, platinum, and tin. In the folio wing- 

 table the values of/ are deduced from the figures of Violle 

 and of Rudberg. The values of k are deduced from the 

 market-prices in London on December 7, 1887 : viz. copper, 

 £75 per ton ; silver, 4s. per troy ounce; iron (rod) £5 10.?. 

 per ton; lead, £16 10s. per ton; platinum, 32s. per troy ounce ; 

 tin, £170 per ton. The figures given in column 6 for the 

 factor of safety for equal cost are calculated by the foregoing 

 formula multiplied by 10 4 . The figures in the seventh column, 

 giving the total cost of equal safety, so far as prime cost of 

 material is concerned, are of course inversely proportional to 

 the figures in column 6. 





/. 



, 



P- 



d. 



k. 



Factor of 

 safety. 



Total cost 

 for equal 



safety. 



Copper ... 



Silver 



Iron 



Lead 



Platinum.. 

 Tin 



1039 



939 

 1585 



307 

 1760 



213-5 



0-0949 

 0-0570 

 0-1138 

 00314 

 0-0325 

 00548 



1615 

 1609 

 9827 



19847 

 9158 



13360 



8-94 

 10-51 



7-79 

 11-35 

 21-36 



7-29 



8-045 

 699-84 

 0-589 

 1-769 

 5598-7 

 18-22 



9-488 

 0-045 

 39 97 

 0-242 

 000052 

 0-6593 



£100 

 £18,770 

 £21 9s. 

 £3508 

 £1.621,600 

 £1287 



The prime cost of copper is known to be greater than that 

 of iron for equal conductivity ; and reckoning on this basis, 

 irrespective of difficulty of fusion, it is stated in the Report of 

 the Lightning-rod Conference to be 50 per cent, dearer. If 

 that were all, then doubtless the small saving in using iron 

 would be more than counterbalanced by the attendant dis- 

 advantages of deterioration by rust, and the like. But when 

 the higher fusion-point, the greater capacity for heat, and the 

 lesser density of iron are taken into account, the superiority 

 of iron, for equal total prime cost, becomes so marked as to 

 once more raise the question whether it may not be expedient to 

 return to the original practice of Franklin, and use lightning- 

 rods of iron instead of those of copper. 



London, December 12, 1887. 



