No. 11. 



Lightning Rods. 



355 



glared in the gloom of the tempest, and rent 

 the rugged oak with its resistless force — and 

 the inventiveness of his genius soon devised 

 the means to prove their identity. Though 

 both Dalibard and Delors appeared to have 

 preceded him in the execution of this inter- 

 esting experiment, yet they only acted upon 

 his suggestion, and with a magnanimity as 

 creditable as it is rare, they awarded to him 

 the full honour of the discovery. 



Having satisfactorily ascertained the iden- 

 tity of cause producing the two sets of 

 phenomena, and referred them to the same 

 governing laws — and being at the same time 

 familiar with the facts explained in No. 1, 

 Propositions 20 and 21, Jig. 3 and 4 — that 

 the electricity of a charged body will always 

 pass to a ball in form of an explosive spark, 

 but that it will pass to a point in a silent and 

 imperceptible manner even at several times 

 the striking distance of the ball — conse 

 quently, that when both are presented as in 

 Prop. 22, fig. 5, the point will discharge 

 the electrified body, and prevent the explo- 

 sion — it immediately occurred to him, that 

 this knowledge might be applied to the pur 

 pose of protecting life and property from 

 injury by lightning. He compared the elec- 

 trified body, fig. 5, to the thunder cloud — 

 the ball to the top of the building, and the 

 point to the lightning-rod, and the parallel 

 was complete. The two arrangements con- 

 sisted of precisely the same series of parts; 

 they were necessarily subject to the same 

 governing laws — and therefore similar re 

 suits must be produced. Thus he arrived at 

 the highly important conclusion, that a pro- 

 perly constructed rod placed in connexion 

 with a building, would silently draw off the 

 electricity from- the cloud, before it could 

 approach near enough to do any injury. 



Every one is aware that the object sought 

 by the employment of lightning-rods, is pro- 

 tection — but there are still very many who 

 do not know how that protection is afl^orded, 

 and ichat conditions are necessary to secure 

 it. They believe that the rod is intended 

 to present an attractive point for the light- 

 ning to strike to, and then to conduct it si 

 lently into the earth. If this was merely 

 an erroneous inference from correct data, it 

 might be allowed to correct itself. The 

 premises however are false, otherwise the 

 reasoning would be just and the conclusion 

 true — for if the rod did possess the power 

 thus ascribed to it, of drawing the stroke, it 

 certainly might sometimes draw it nearer 

 to them and thus do injury, when it would 

 otherwise have passed at a harmless dis 

 tance. But from what has been shown, it 

 appears that a pointed conductor never can 

 draw a stroke — only a silent and harmless 



stream. I do not hesitate to repeat the 

 strong language of Observer, " that it is ab- 

 solutely impossible for the lightning to 

 strike to a conductor which is properly 

 constructed in all its parts." 



1. The material and construction of the 

 rod. — There is so much diversity of opinion 

 among teachers and writers upon the sub- 

 ject, that the inquirer may often despairingly 

 exclaim — "who shall decide when doctors 

 disagree?" Thus Dr. Comstock says, "as 

 the electric fluid occupies the surface of 

 bodies only, the most perfect lightning-rodg 

 are made of copper tubes, left open so as to 

 conduct on the inner as well as the outer 

 surface." J. M. C. says, " the larger the 

 rod the greater the security, as the conduct- 

 ing power is in proportion to the solid 

 mass." Professor Olmstead quotes authori- 

 ties to support the latter opinion, and then 

 adds, " it is admitted that the fluid pervades 

 only the surface — still it is maintained that 

 the conducting power depends on the mass." 



Again — Dr. Comstock says, " lightning' 

 rods should not be painted — oil beiog abso- 

 lutely a non-conductor of electricity." J. M. 

 C. suys, " the rod should be well painted 

 with several coats of black paint, which not 

 only protects it from moisture, but also 

 tends to increase its conducting power." 

 Professor Olmstead says, "a coating of black 

 paint, the basis of which is charcoal, is es- 

 sential to the permanent eflicacy of the 



1 will not go into the inquiry whether the 

 electricity in its passage pervades the sur- 

 face or the substance of the rod, because it 

 is unnecessary to do so. It has been shown, 

 Props. 17 and 32, figs. 2 and 15, that teh 

 fluid is attracted with a force proportioned 

 to the quantity of matter contained in the 

 charged body. The solid rod containing 

 more matter than the tube, it will exert a 

 stronger attractive force, and will conse- 

 quently hold a larger quantity of the fluid 

 within the sphere of its attraction. Hence it 

 is practically true that the conducting power 

 is in proportion to the mass of the rod. From 

 Prop. 31, fig. 14, it would also appear, that 

 the tube recommended by Dr. Comstock, 

 could not conduct on the inner surface, for 

 the simple reason, that the strong mutual 

 repulsion of the electrical particles would 

 not allow them to enter. A solid rod of 

 sufficient size is therefore to be preferred. 



The question of paint may soon be dis- 

 posed of. As the point of the rod only de- 

 rives the electricity from the cloud, its at- 

 tracting power cannot be affected by either 

 a coating of paint or rust upon the surface 

 below. And so long as it remains to be the 

 case that the electric fluid will freely pass 



