254 



SCIENCE. 



[Vol. XVII. No. 431 



pers on Electricity and Magnetism," and I found that he 

 described in detail the case of a farmhouse in Scotland, which 

 •was struck by lig-htning, and in which this very dissipating 

 •eflEect took place ; that is, the bell-wires were dissipated, — 

 an occurrence which, as you know, is extremely common 

 when a lightning-discharge takes place. I went on through 

 the records, and found numberless cases of this, the oldest 

 "being that of the dissipation of the metal covering on the 

 "wooden shield of some Greek warrior. I mention this case 

 as of interest, as it brings out a very fortunate circumstance, 

 that when thin metal is dissipated against wood or even 

 against plaster, no harm results to the wood or plaster. Of 

 course, you know that it has been somewhat discussed whether 

 this action is a dissipation through the heating of the metal, 

 or whether it is a cold dissipation — a breaking-up into par- 

 ticles, as it were — of the metal. On this point I have noth- 

 ing to say. 



But as I went on through the records I could not make 

 the facts accord satisfactorily with my hypothesis. The dis- 

 sipating action that I was looking for certainly took place, 

 and is a very common accompaniment of lightning-dis- 

 charges, but in spite of it, there was damage to the building. 

 It was only after a considerable reading of the records that 

 it gradually dawned on me that I had found no case where 

 damage to the building occurred on the same level with the 

 dissipated conductor. 



Let me describe here in Franklin's own words a typical 

 case of the action of a small conductor dissipated by the dis- 

 charge. 



Franklin, in a letter to CoUinson read before the Royal 

 Society, Dec. 18, 1755, describing the partial destruction by 

 ilightning of a church-tower at Newbury, Mass., wrote: 

 '■"Near the bell was fixed an iron hammer to strike the hours; 

 and from the tail of the hammer a u-ire went down through 

 a small gimlet-hole in the floor that the bell stood upon, and 

 through a second floor in like manner; then horizontally 

 Tinder and near the plastered ceiling of that second floor till 

 it came near a plastered wall; then down by the side of that 

 •wall to a clock, which stood about twenty feet below the 

 liell. The wire was not bigger than a common knitting- 

 needle. The spire was split all to pieces by the lightning, 

 and the parts flung in all directions over the square in which 

 the church stood, so that nothing remained above the bell. 

 The lightning passed between the hammer and the clock in 

 the above mentioned wire, without hurting either of the 

 ioors, or having any effect vipon them (except making the 

 gimlet-holes, through which the wire passed, a little bigger), 

 and without hurting the plastered wall, or any part of the 

 building, so far as the aforesaid wire and the pendulum-wire 

 of the clock extended; which latter wire was about the thick- 

 ness of a goose-quill. From the end of the pendulum, down 

 quite to the ground, the building was exceedingly rent and 

 damaged. . . . No part of the aforementioned long, small 

 wire, between the clock and the hammer, could be found, 

 except about two inches that hung to the tail of the hammer, 

 and about as much that was fastened to the clock ; the rest 

 being exploded, and its particles dissipated in smoke and air, 

 as gunpowder is by common fire, and had only left a black, 

 smutty track on the plastering, three or four inches broad, 

 darkest in the middle, and fainter towards the edges, all along 

 the ceiling, under which it passed, and down the wall." 



I would thus formulate what seems to be true, — that a 

 conductor which can be easily dissipated by a lightning-dis- 

 charge protects the building to which it is attached between 

 two horizontal planes, the one passing through the upper 



end of the dissipated conductor, and the other through the 

 lower end; and it is this one point that I would urge upon. 

 the consideration of the Institute. 



I have taken the time of the Institute to tell how I reached ■ 

 this conclusion; but, as must always be, I reached it by 

 making some false digressions. So far as I know, therefore, 

 a conductor such as I have here — a conductor made of 

 light copper ribbon, so that seventy-five feet of it will weigh 

 only a pound, and made in sections two feet long, which 

 shall be tacked to the building from its ridge-pole to the 

 foundation, the joints being made of low conductivity by 

 the insertion of insulating washers — will protect the build- 

 ing. The conductor will be destroyed by the discharge. Its 

 destruction can take place even against a plastered wall 

 without injury to the wall; but no other harm will occur so 

 far as the conductor extends in a vertical direction. There 

 is no need of the conductor following the shortest course to 

 the ground. There is no need of providing a good earth 

 connection. I can see no difference between the two ends of 

 the metallic ribbon. You do not attempt to make a good 

 connection at the top with the dielectric, and I do not see 

 why you should attempt to make a good connection at the 

 bottom. In no case on record of the protecting influence of 

 dissipatable conductors has this protecting influence depended 

 upon there being a good earth connection. Of course, the 

 ribbon should not be boarded over. Free gun-powder burns 

 harmlessly enough, but it causes damage when burned in a 

 confined space; and the dissipation of a conductor presents 

 similar phenomena. 



It would not do to run such a conductor as I suggest here 

 part way down the building, and then make it turn up again 

 before its final descent to the ground, as in such a case there 

 would probably be a line of disaster from the point where 

 the upward turn began. 



Doubtless numerous improvements can be suggested, but 

 letting this stand as the main point of what I have to say, — 

 that a dissipatable conductor protects, — it may be of interest 

 to consider why it protects. But here you will understand 

 perfectly well that, while I can offer certain explanations 

 which seem fairly plausible to me, it is not in the nature of 

 things that I should have gotten at the whole truth. 



In order to destroy a building in whole or in part, it is 

 necessary that work should be done; that is, energy is re- 

 quired. Just before the lightning-discharge takes place, the 

 energy capable of doing the damage which we seek to pre- 

 vent exists mainly in the column of air extending from the 

 cloud to the earth in some form that makes it capable of 

 appearing as what we call electricity. We will therefore 

 call it electrical energy. What this electrical energy is, it is 

 not necessary for us to consider; but that it exists there can 

 be no doubt, as it manifests itself in the destruction of build- 

 ings. 



The problem that we have to deal with, therefore, is the 

 conversion of this energy into some other form, and the ac- 

 complishment of this in such a way as shall result in the 

 least injury to property and life. When lightning-rods were 

 first introduced, the science of energetics was entirely unde- 

 veloped ; that is so say, in the middle of the last century, 

 scientific men had not come to recognize the fact that the 

 different forms of energy — heat, electricity, mechanical 

 power, etc. — were convertible one into the other, and that 

 each could produce just so much of each of the other forms, 

 and no .more. The doctrine of the conservation and corre- 

 lation of energy was first clearly worked out in the early 

 part of this century. There were, however, some facts known 



