CHAP. II.] 



LIGHTNING-CONDUCTORS. 



539 



but this precaution would be quite insufficient if it were not com- 

 bined with the principal one of a well into which the conductor dips 

 by several branches, as is shown in Fig. 345. When a lateral branch 

 is put in communication with the soil, it should be surrounded by 

 charcoal, which is at the same time a good conductor of electricity 

 and a preservative against rust. 



Numerous facts prove the effi- 

 cacy of lightning-conductors, but 

 for this efficacy to be real, the 

 apparatus must fulfil all the con- 

 ditions above enumerated. The 

 number also of the lightning- 

 conductors and the height of the 

 rods must be determined by the 

 dimensions of the buildings they 

 are designed to protect. Expe- 

 rience has shown that the greater 

 the height of the rod above the 

 ridge of the building, that is 

 above its junction with the con- 

 ductor, the wider is the range of 

 its protective power. The radius 

 of its range is about twice the 

 height of the rod. These facts 

 enables us to determine the num- 

 ber of lightning-conductors which 

 must be set upon a house or other 

 building. The rule, according to 

 Arago, may be stated as follows : 

 " The smaller the height of the 

 rods the more of them there 

 should be. Their number will 

 be sufficient, provided that no point on the top or on any terrace is 

 at a greater horizontal distance from the nearest rod than double 

 the height of that rod above its base." 



Vertical lightning-conductors are sufficient when the building is 

 not of great height, when it is, the sides must be specially protected, 

 for there are instances of buildings struck by lightning at points far 



FJG. 345. The fixing of lightning-conductors. 

 Vertical and oblique rods. 



