SMITHSONIAN MISCELLANEOUS COLLECTIONS. 



237 



DIRECTIOiv^S FOR CONSTRUCTING LIGHTNING-RODS. 



From Essays on Meteorologt, by Prof. Joseph Henry. 



1st. The rod should consist of round iron, of not less than 

 three-fourths of an inch in diameter. A larger size is preferable 

 to a smaller one. (Ordinary gas-pipe may be employed.) Iron is 

 preferred, because it can be readily procured, is cheap, a sufficiently 

 good conductor, and, when of the size mentioned, cannot be melted 

 by a discharge from the clouds. Other forms of rod, such as flat 

 or twisted, will conduct the lightning, and in most cases answer 

 sufficiently well. They tend, however, to give off lateral sparks 

 from the sharp edges at the moment of the passage of the elec- 

 tricity through them, which might, in some cases, set fire to very 

 combustible materials. 



od It should be, through its whole length, in perfect metallic 

 continuity; as many pieces should be joined together by welding 

 as practicable, and, when other joinings are unavoidable, they 

 should be made by screwing the parts firmly together by a coupling 

 ferule, care being taken to make the upper connection of the latter 

 with the rod water-tight by cement, solder, or paint. 



3d. To secure it from rust, the rod should be covered with a 



coating of black paint. . . +i „ „^„^ 



4th It should be terminated above with a single point, the cone 

 of which should not be too acute, and to preserve it from the wea- 

 ther, as well as to prevent its being melted, should be encased with 

 plat num, formed by soldering a plate of this metal, not ess than 

 a twentieth of an inch in thickness, into the form of a hollow cone 

 Points of this kind can be purchased of almost any mathematical 

 instrument maker. Usually the cone of platinum, for convenience 

 s first attached to a brass socket, which is secured on the top of 

 the rod, and to this plan there is no objection. The platiauo. 



