438 ENGINEERING ON THE FARM 



Theory of lightning rods. The theory of lightning rods is 

 based on the principle that to protect buildings from light- 

 ning stroke it is necessary to make an easy path for the cur- 

 rent from the highest point on the building into the moist 

 earth. Experience has shown that some of the conditions 

 which were supposed to be essential when lightning rods 

 were first invented are not necessary. At one time the 

 metal conductors were insulated from the building by the 

 use of glass in an effort to keep the entire lightning-rod sys- 

 tem from coming into contact with any part of the building. 

 It has been proved that this is a mistaken idea and that the 

 object of the lightning rod is to maintain connection between 

 the building and the moist earth and to carry away the 

 electric current as rapidly as it may accumulate in any part 

 of the building, as well as to take it direct from the air. 



Material. The material used for conductors should be a 

 substance which will offer a minimum resistance to the pas- 

 sage of the current and will not be destroyed by rust or 

 corrosion. At the present time two materials are used: 

 copper and galvanized iron. Copper is the better conductor 

 and does not readily deteriorate from the action of the 

 elements, but it may cause side-flashes and is quite expen- 

 sive. Iron offers greater resistance to the current, but is 

 much cheaper and is preferred to copper by some authorities. 

 When it is galvanized or protected by a coat of aluminum 

 paint, its durability is satisfactory. Where copper is used, 

 the conductor is usually made of woven copper fabric rather 

 than a rod, because greater conductivity can be secured by 

 the use of the fabric than if the same weight of metal is 

 used as a rod. If the conductor is small, there is danger of 

 its being melted by the current. The larger it is, the greater 

 the expense becomes, the cost being the only objection to a 

 large conductor. The conductive qualities of different fab- 

 ricated forms of the same metal can be determined by the 

 weighing of equal lengths. The heavier contains more metal 

 in a given length and is the better conductor of the two. 



