32 



Lightning Rods, JVo. 3. 



Vol. X. 



liightning Rods, No. 3. 



(Continued from page 356, last volume.) 



4. The point of the Rod. — It is necessary 

 that a lightning rod should have an acute 

 slender point, not liable to be destroyed by 

 rust — hence it has been found expedient to 

 employ some metal which is not liable to 

 rust for this purpose. PJatina, gold, and 

 silver, are the most proper. The cost of 

 the two former renders them somewhat ob- 

 jectionable, and their place can be very well 

 supplied by the latter, at a much less ex- 

 pense. A half dollar cut into three or four 

 slips, and then forged into square tapering 

 points about two inches long, with a screw 

 tap at the lower end to attach them to the 

 rod, will make so many very good points 

 when neatly finished. This may be done 

 by any ordinary blacksmith, taking care not 

 to heat the metal quite to a red, so as to 

 make it crumble under the hammer. Very 

 good points may thus be obtained for twenty- 

 five cents each, instead of paying some ten 

 or twenty times that sum for those which 

 are perhaps not as good. With a view to 

 improve the quality and appearance of the 

 rod and to make it less top-heavy, it is ad- 

 visable to draw three or four feet of the 

 upper end to a taper, so that the top shall 

 correspond with the base of the point. A 

 hole should then be drilled in the end — a 

 female screw cut in it, and the point firmly 

 screwed in. I have had points of this de- 

 scription in use for fifteen years, and do not 

 perceive that they have sustained any in- 

 jury. 



It has long been a controverted question, 

 whether one point or several to a rod is to 

 be preferred. Professor Olmstead says, "Ac- 

 cording to the experiments of Earl Stanhope, 

 made more than sixty years ago, a single 

 needle will discharge a Leyden jar more 

 rapidly than a bundle of the same." " I 

 believe one point -preferable to several, es- 

 pecially when they diverge from each 

 other y Observer says, " There is reason 

 to doubt whether much advantage is gained 

 by increasing the number of points, unless 

 they are separated to a considerable dis- 

 tancey Thus Stanhope found one prefer- 

 able to several, when they did not diverge. 

 Olmstead believes the same to be true when 

 they do diverge; and Observer agrees with 

 them unless when they are separated to a 

 considerable distance. There is little dis- 

 crepancy in all this. Stanhope's bundle of 

 needles taken collectively, made a blunt in- 

 stead of a sharp point — each one was cir- 

 cumstanced with regard to the others as the 

 point in fig. 6,* is to the contiguous body. 



See page 249, last vol. of Cabinet. 



I understand Observer to mean, that the 

 points should be separated to such a dis- 

 tance as not to interfere with each other's 

 action. Thus each point being effective, 

 they will certainly discharge a cloud more 

 rapidly than one alone could do. This 

 seems to be a plain common-sense view of 

 tlie subject. Unless the points can be scat- 

 tered over distant parts of the building re- 

 mote from each other, they will hardly com- 

 pensate for the cost. And when so placed 

 it becomes a matter of convenience — a ques- 

 tion of economy, whether the several points 

 shall all be connected with the same rod or 

 with different ones. 



5. The height of the Rod.— I find it will 

 be impossible to explain the operation of 

 points under the varied circumstances in 

 which they are placed, without the aid of 

 further illustrations, and this seems denied 

 to me. Jt must therefore suffice to say, that 

 the greater the elevation of the rod, the more 

 extensive will be its influence. Observer 

 has well remarked, " that the length of the 

 rod should be such as to elevate its point as 

 far as possible above the top of the building 

 and every other contiguous object." Every 

 near object, whether lateral to, or beneath 

 the point, must impair its operation in some- 

 what the ratio of its distance and magnitude. 

 A rod 50 feet long erected on an extended 

 plane, would not be as effective as if it were 

 placed on the top of a high conical hill. So 

 also a rod raised from the centre of a flat 

 roof, will possess less protective power than 

 if placed on the C07nb of a common roof. 

 When a rod is overtopped by an adjacent 

 building, chimney, tree, &c., it is rendered 

 comparatively inert. — (See prop. 23, 24,_^^s. 

 6, 7.) Protection is the primary object — we 

 desire the greatest possible security. I would 

 therefore advise in every case to raise the 

 rod as high as it can be supported. If this 

 is thought to be still too indefinite, I would 

 say let it rise at least ten feet above every 

 near object. Professor Olmstead, when 

 speaking of " vanes, balls and ornaments, 

 which are oflen placed on rods," says, "I 

 think these appendages do not generally 

 affect the eflicacy of the rod." Observer 

 says, " it is found that all objects when near 

 to the point, diminish its effects in a rapidly 

 increasing ratio to their height, until they 

 render it entirely useless." The sentiment 

 of Observer is confirmed by the experiment 

 shown fig. 7. It is therefore not advisable 

 to connect any such appendages with light- 

 ning rods. They must, to a certain extent, 

 frustrate the intention for which they were 

 constructed, and may in extreme cases lead 

 to disastrous consequences. 



6. The extent of Protection. — Here, too, 



