-1859J WRITINGS OF JOSEPH HENRY. 387 



The first requisite is found in the tip of an iron rod gilded, to 

 prevent its becoming blunted by rust ; but a point of this 

 kind, though it may protect a building from the first dis- 

 charge which strikes it, will be melted, and the intensity of 

 its action thereby diminished in the case of a subsequent 

 explosion. At the upper termination of the lightning-rod, 

 a small cone of platinum attached to a copper socket which 

 fits on the top of the rod, made conical for that purpose, is 

 now usually employed. Tips of this kind are now generally 

 offered for sale in the large cities. The quantity of platinum 

 on them however is generally too small, since we have known 

 them in several instances to be fused by a discharge of light- 

 ning. The point itself should be the apex of a solid cone of 

 platinum or of a thick plate of that metal, fastened by screw- 

 ing or soldering to the copper socket. 



We frequently see announcements in the papers of great 

 improvements in lightning-rods, for which patents have been 

 obtained, and among these boasted improvements have been 

 the application of magnetized steel points to receive the 

 lightning ; but this invention, like most of the others which 

 have been given to the public for the same purpose, is the 

 result of some imaginary analogy, or of sheer charlatanism. 

 It rests upon no foundation of observation, experiment, or 

 theory. The magnetization of a bar, so far as it has any effect, 

 tends to cause the electrical discharge to revolve around it, 

 and to render the iron very slightly, if anything, a less per- 

 fect conductor. 



The horizontal distance from a rod to which the pro- 

 tecting influence extends, is a question of considerable im- 

 portance. It has generally been admitted that the point 

 of a lightning conductor protects a horizontal circular 

 space with a radius equal to twice its own height ; that is, 

 if the elevation of a rod above a flat roof be ten feet, it will 

 protect a circular space of twenty feet radius, or forty feet 

 diameter. But this rule cannot always be depended upon ; 

 for although it may be true in regard to buildings of stone 

 or brick, with an ordinary sloping roof covered with tiles or 

 slate, it would scarcely hold good if considerable masses of 



