PROTECTION FROM LIGHTNING. 53 



which so far as can be learned from the Bible or from Josephus, 

 remained unscathed by the lightnings of Ileaven during a period 

 of more than a thousand years. In the structure of its roof, in 

 its covering with a coating of gold ; in its long, pointed, gilt iron 

 or steel lances, projecting from its roof; in its walls overlaid with 

 wood thickly gilt, and in its metallic rain pipes connecting the 

 roof with cisterns for water in the courts of the Temple; presented 

 the most ample provision for the ready conduction of electricity ; 

 in fact, lightning rods very similar to those employed at the 

 present day. 



It is a remarkable and interesting fact, that tall pyramidal spires 

 dating back into the middle ages — spires which have been re- 

 peatedly struck by lightning — on being furnished with lightning 

 conductors within the last century, have had entire immunity from 

 such strokes. 



The lightning rod already devised, with reference to size, form, 

 point and connection with the earth, should be noticed more 

 definitely. 



1st. The rod may consist of iron or copper — the former metal 

 being generally preferred from considerations of expense, and for 

 its greater firmness when set, by which it withstands the action of 

 winds. It should be at least three-fourths of an inch in diameter 

 for buildings of ordinary height. For very high structures it 

 should be made larger. In form it is usually round, although 

 other forms are used and serve a good purpose. A copper rod 

 has the advantage of high conducting powers for electricity, its 

 ratio to iron in this repect being as six to one ; but it is want- 

 ing in stiffness, and is considerably more expensive than iron.* 



2nd. To secure the rod from rust, it should be coated with 

 paint containing lampblack. Galvanized iron is a good substitute 

 for the painted rod, inasmuch as the zinc coating has a conducting 

 capacity for electricity nearly double that of iron. 



3d. Metallic continuity should be perfectly maintained through- 

 out the entire length of the rod. So far as practicable the parts 

 should be joined together by welding. When other joinings can- 



— . ■ — ■ m 



*Thc following table contains the electrical conductivity of several of tbc tnoro com- 

 monly known metals at 3'2° Fah., compared with silver as a standard. The results were 

 obtained by Matshierren by a series of careful experiments. 



Silver 100.0 Platinum 18.0 



Copper 99 9 Iron 16.8 



Gild 80.0 Tin 13.1 



Zino 29.0 Lead 8.3 



