CONDUCTORS. 271 



better conductor than any other in its neighbour- 

 hood, if, besides, it towers up above them, and 

 yet is in excellent conducting connection with the 

 moist ground, the flash, when it strikes, will most 

 likely choose it out before all other objects. 



By the term liglitning -conductor, we mean such 

 a conductor as we may be certain will present itself 

 as a channel before any other object near it, for 

 the passage to each other of the electricities of the 

 earth and cloud. The conditions which are espe- 

 cially requisite in a lightning-rod, are at once 

 evident from a due consideration of the known 

 properties of electricity. The metals are the fittest 

 materials for the purpose, since they surpass, 

 beyond all comparison, every other substance in 

 conducting power. Copper, again, would be 

 chosen as the best conductor among metals ; iron, 

 however, is commonly preferred on account of its 

 cheapness. 



This metallic conductor must now be placed so 

 as to jut up above the highest point of the building 

 to be protected, and must run down in unbroken 

 metallic connection to the moist earth or to running 

 water, presenting to these the greatest possible 

 number of points of contact, so as to favour the 

 escape of the electricity. Throughout its whole 

 length, too, it must be thick enough to offer less 

 resistance than any other body near it, to the 

 passage of the greatest possible quantity of elec- 



