thp: chambers lightning-rod. 43 



qual tension are usually within striking distance is amply proved by the 

 fact above mentioned — that almost all the electric discharges occur 

 between cloud and cloud. But that this is not always true is proved 

 with equal certainty by the other fact that sometimes the discharge 

 takes place between the cloud and the earth. Against the former we 

 need no protection, because discharges between masses of air of une- 

 qual tension are harmless to the earth and things on it. It is the latter 

 kind of discharge solely against which lightning-rods are intended to 

 afford protection. The assumption, therefore, that a mass of air over- 

 charged is always near enough to one that is undercharged to flash into 

 it being baseless, renders any mode of protection founded upon it to a 

 great extent untrustworthy. I say to a great extent, because a mass 

 of overcharged air, though naturally out of reach of a mass of under- 

 charged air, may be brought within reach of the same by artificial 

 means — and this is what is attempted by the Chambers lightning-rod. 

 A cloud or over-excited mass of air at one place may be out of strik- 

 ing distance of a mass of under-excited air,^ but if a long copper rod 

 be laid from one to the other, or from near the one to near the other, 

 the spark may pass. Consequently, if a large or long building be fitted 

 with the Chambers rod, with no earth connection, it is more than prob- 

 able that some of the most distant points will be beyond the influence 

 of a thunder-cloud that is able to discharge into a nearer one, and may, 

 consequently, take the charge and pass it off" into the less electrically 

 tense air that overhangs them. In this way, a building so protected 

 may be struck and not injured. It is, however, obvious that this pro- 

 tective power must rapidly diminish with the size, and especially with 

 the length, of the structure; and when this is reduced to small dimen- 

 sions, it is more than possible that all the points are within striking dis- 

 tance of the same cloud. Consequently, there is then no discharge, 

 and the danger to the building is vastly increased by the presence of 

 the rod. 



Now, the Phoenix Mill was a structure of this kind. It measured 

 only fifty by thirty feet, and all its points were probably within the in- 

 fluence of the mass of excited air from which the flash proceeded. 

 Not having any path provided for its escape, it took the conductor and 

 then the building in its passage to the earth. It is needless to add 

 that, on this view, an ordinary conductor, with good earth-plate, would 

 have efficiently protected the building. 



It may be replied that the conductor was insulated from the roof; 

 but the reply would be futile. It matters little or nothing whether this 



