May 15, 1891.J 



SCIENCE. 



269 



tion from the induced currents produced by the oscillating 

 discharge in neighboring conductors. 



This, perhaps, may account for some of those phenomena 

 mentioned to-night: that, when the lightning-rod is dissi- 

 pated, that is when its resistance was very high in compari- 

 son with the quantity of electricity rushing through, there 

 was a steady discharge and no harm was done ; while, 

 when it is an oscillating discharge, the slightest irregularity 

 ■will cause the discharge to "jump the track," that is, to 

 leave the lightning-rod, which is obstructed by the counter 

 electro-motive force of self-induction, and to spark over to 

 metal masses of larger condenser capacity : for what I 

 consider as the most dangerous part of lightning discharges 

 is not the enormous voltage of the discharge, nor the strong 

 current rushing through the lightning-rod, but the electro- 

 magnetic field of force, which alternates with enormous fre- 

 quency and reaches far out into space from the real path or 

 centre of disruptive discharge, and thereby must cause in- 

 ductive effects everywhere, which, as before said, cause not 

 only the main discharge to spark over, but produce true sec- 

 ondary or induced lightning discharges. Hence I must be 

 very much in favor of every arrangement which is able to 

 change the oscillating discharge into a steady rush of cur- 

 rent. 



The resistance of the lightning-rod I consider as of sub- 

 ordinate importance only, except so far as carrying capacity 

 is concerned : for of what use is a resistance as low as a 

 fevr ohms, when the self-induction of the lightning-rod causes 

 a spurious resistance of perhaps hundreds of thousands of 

 ohms ? 



Mr. Hodges: — -I would bring this discussion back once 

 more to this matter of fact that I am interested in. The 

 theory I do not care so much about. It may be interesting 

 as mental gymnastics. I came here feeling quite sure that 

 somebody would stand up and say, " I know a church or a 

 house in this town or that town where the conductor was 

 dissipated and yet damage was done on the same level." I 

 have not found a case. 



Dr. Geyer: — In a disruptive discharge, the length of the 

 lightning-rod, it seems to me, is a very small part of the total 

 path. I should imagine that any resistance the conductor 

 would have would be such a small part of the total that it 

 would not have much effect on the character of the discharge. 



Mr. Steinmetz: — -I believe I have been misunderstood in 

 what I meant by the influence of the resistance upon the na- 

 ture of the discharge. Indeed, the whole resistance of the 

 lightning-circuit is so large that under any circumstances the 

 resistance of the lightning-rod is imperceptibly small. But, 

 as explained in my former remark, it is not the resistance 

 proper, but the consumption of energy by the resistance, 

 which causes the amplitude of the oscillating discharge to 

 decrease slower or quicker until, for a very rapid consump- 

 tion of energy by resistance, only one wave appears that is 

 a steady or continuous current. This phenomenon is 

 ■similar to a pendulum oscillating in a liquid : the greater 

 the frictional resistance of the liquid, the quicker the am- 

 plitude of the pendulum motion decreases, until, at 

 last, in a very tough liquid, the pendulum comes to 

 rest without any oscillation at all — periodically. In such 

 a way the resistance of the conductor, by consuming 

 the energy of the electric discharge, could change the dis- 

 charge from an oscillating to a continuous one, although 

 the whole "resistance" has still about the same value, 

 *' infinite," if we were allowed to speak with the usual mean- 

 ing of " resistance" of disruptive discharges, which we are not. 



Mr. Birdsall: — I think Mr. -Hodges has given us the most 

 original idea on this lightning-rod question that has been put 

 forward for some time. I also think that Mr. Steinmetz has 

 hit the nail on the head in his explanation of it. It only 

 shows us again what we do not know about the various phases 

 of alternating currents. His theory also gives me a little 

 uneasiness, because I have advised a number of friends who 

 have built houses in the country to put in a metal lath, as I 

 thought fehat, having plenty of metal around, if the house 

 happened to be struck, it would go to the ground through 

 this metal lath. Now, if any of those houses are struck, 

 and that metal lath turns into gas, I think I shall emigrate. 



Mr. Hodges: — That metal lath reminds me, — I wrote to 

 Edward Atkinson about this. You know he is president of 

 about the only insurance company in the country that cares 

 about stopping fires; that is, reducing the amount of dam- 

 age done. He wrote back that they had experience with 

 lightning-rods, and that their experience was such that they 

 had abolished them on all factories that were insured by the 

 Manufacturers' Mutual Fire Insurance Company. Now, in 

 the mills there is a considerable surface of metal; and they 

 find, as is natural, that the discharge spreads itself probably 

 over the surface of this metal. At any rate, the potential 

 was so reduced as to very materially mitigate the effects. 

 As Mr. Atkinson puts it, it spreads out over the surface of 

 the machinery, so that no great damage is done. But they 

 have taken off their old rods. 



Mr. Wolcott : — There is one question I would like to ask 

 in regard to that drawing on the board. If you do not say 

 that no damage was done to the end of the building, in spite 

 of the fact that the conductor was dissipated, why don't you 

 have to say that no damage was done along the eaves, in 

 spite of the fact that the conductor was not dissipated ? 



Mr. Hodges : — That is a fair question. A dissipated con- 

 ductor may run horizontally any reasonable distance, and 

 then run down ; and when it goes to pieces the thing is saved. 

 But when the conductor is not dissipated, there are any 

 number of cases where the building is not saved. 



Mr. Wolcott: — I can understand it, that a dissipating con- 

 ductor would very often save the building, but, according to 

 the accounts that have been cited, it does not seem to make 

 any difference how little there is of that metal. There must 

 be some limit. When it gets down where a little bit of gold- 

 leaf is going to save a building, it looks rather improbable. 

 If a little bit of metal being dissipated would save a building 

 from a lightning discharge, then an ordinary lightning dis- 

 charge would not be sufficient to dissipate some of these 

 larger conductors which are dissipated. 



Mr. Hodges: — I do not pretend to understand any thing 

 about it. I have theorized upon it, but that is not impor- 

 tant. It is only the fact, and the fact stands there until 

 somebody gets up and shows a specific case where it does not 

 work. 



Mr. Birdsall: — I do not think that Mr. Wolcott can hold 

 that argument, because he has not any data on the compara- 

 tive energies of these various discharges of which we have 

 record. We have a record of the damage done in the dissi- 

 pation of the conductor, but we have no record of the foot- 

 pounds of energy in the discharge. 



Now, the discharge t^iat burnt up the gold-leaf on the wall 

 might have been a great deal smaller than some of the dis- 

 charges which burned up the larger conductors. Then an- 

 other point has been raised about the replacing of the con- 

 ductor immediately after it was dissipated. This will never 

 be necessary, it seems to me, for it is a recorded or alleged 



