332 On Lightning- Rods. 



mense surface of the earth, will retain no sensible portiort 

 of it. 



A lightning-rod is defined to be a conductor which the 

 electric matter prefers to the surrounding bodies, in its de- 

 scent to the ground for the purpose of expanding itself, and 

 eommonly consists of a bar of iron elevated on the buildings 

 it is intended to protect, and descends, without any divisions 

 or breaks in its length, into water or nioist ground. When 

 the rod is not perfect in its comnrsunication with a moist soil, 

 or has breaks in it, the lightning, in its course, leaves it at 

 that spot, for some other near body, or divides itself between 

 the two to pass more rapidly into the earth. 



It is proved by the experiments of MM. de Romas and 

 Charles, that the higher ihe rod is elevated in the air, other 

 circumstances being equal, the more its efiicacy will be in- 

 creased. It is announced that (he most advantageous form 

 for the extremity is that of a very sharp cone. In this coun- 

 try it is usual to have three points diverging — in Europe, on 

 the continent particularly, only one is used, placed perpen- 

 dicularly. 



How far the sphere of action of the rod extends has not 

 been accurately determined ; but it is known that some 

 buildings have been struck even when they had rods attach- 

 ed to them. This however has always taken place at a 

 certain distance from the conductor — say 3 or 4 times its 

 length.* It is the opinion of M. Charles, that a rod will 

 effectually protect from lightning a circular space, whose 

 radius is twice that of the height of the conductor. Bynn- 

 creasing the height of the conductor, therefore, the space 

 protected will be enlarged in proportion. 



A current of electric matter, whether luminous or not, is 

 always accompanied by heat, the intensity of which depends 

 upon the velocity of the current. This heat is sufficient to 

 make a metallic wire red hot, or to fuse or disperse it, if 

 sufficiently small, so that thin slips of copper nailed to the 

 masts of vessels afTord no security. The heat of the elec- 

 tric fluid scarcely alters the temperature of a bar of metal, 

 on account of its mass ; and no instance has yet occurred of 

 a bar, of rather more than half an inch square, or of a cylinder of 

 the same diameter, having been fused, or even heated red hot 



* The length of tlie stem, aud not of the whole rod, is undoubtedly 

 here intended, C. H. 



