July 25, 1890.] 



SCIENCE. 



47 



currents keep entirely to the outer skin, penetrating only a frac- 

 tion of a millimetre into iron, and they make this skin intensely 

 hot. But the central core keeps cool until conduction has time to 

 act; and consequently, unless the wire is so thin as to be bodily 

 deflagrated by the discharge, its continuity is not likely to be inter- 

 rupted. Thickness of wire is thus more needed in order to resist 

 ordinary deterioration by chemical processes of the atmosphere 

 than for any other reason. 



But the liability to intense heating of the outer skin should not 

 be forgotten, and care should be taken not to take the wire past 

 readily inflammable substances for that reason. For instance: it 

 would be madness to depend on Harris's notion that a lightning- 

 conductor through a barrel of gunpowder was perfectly safe, 

 especially if said conductor were an iron wire or rod. 



In the old days a lightning-conductor of one or two hundred 

 ■ohms resistance was considered dangerously obstructive, but the 

 impedance really offered by the best conductor that ever was 

 made to these sudden currents is much more like 1,000 ohms. A 

 column of copper a foot thick may easily offer this obstruction, 

 and the resistance of any reasonably good earth connection 

 becomes negligible by comparison. A mere wire of copper or 

 iron has an impedance not greatly more than a thick rod, and the 

 difference between the impedance of copper and iron is not worth 

 noticing. 



But although, in respect of obstructing a flash, copper and iron 

 and all other metals are on an approximate equality, it is far 

 -otherwise with their resistances, on which their powers of dissi- 

 pating energy into heat depend. It is generally supposed that iron 

 resists seven times more than copper of equal section, and so it 

 does steady currents; but to these sudden flashes its resistance is 

 ■often a hundred times as great as copper, by reason of its magnetic 

 properties. This statement is quite reconcilable with the previous 

 .-statement, that in the matter of total obstruction there is very 

 little to choose between them: the apparent paradox is explicable 

 l)y the knowledge that rapidly varying currents are conveyed by 

 the outer skin only of their conductor, and that the outer skin 

 available in the case of magnetic metals is much thinner than in 

 :the case of non magnetic. 



Questions about shape of cross- section are rather barren. Thin 

 tape is electrically better than round rod , but better than either is 

 a. bundle of detached and well separated wires (for instance, a set 

 ■of four, one down each cardinal point of a chimney); but it is easy 

 to overestimate the advantage of large surface as opposed to solid 

 contents of a conductor. The problem is not a purely electrical 

 one: it is rather mixed. The central portion or core of a solid rod 

 is electrically neutral, but chemically and thermally and mechan- 

 ically it may be very efficient. It confers permanence and 

 strength; and the more electrically neutral it is, the less likely is it 

 to be melted. Its skin may be gradually rusted and dissolved off, 

 or it may be suddenly blistered off by a flash; but the tenacity of 

 the cool and solid interior holds the thing together, and enables 

 it to withstand many flashes more. Very thin ribbon or multiple 

 "Wire, though electrically meritorious, is deficient io these cotmnon-- 

 3)lace advantages. 



There were two functions attributed to high conducting power 

 in the old days, — first, the overpowering of all other paths to earth ; 

 second, the avoidance of destruction by heat. The first we have 

 :seeii to be fallacious: on the second a few more explanations can 

 'be made. In so far as fusion by simple current strength is the 

 thing dreaded, it must tee noticed that a good conductor has no 

 igreat advantage over a bad conductor. It is a thing known to 

 junior classes, that, when a given current has to be conveyed, less 

 .heat is developed in a good conductor, but that, when an eleotro- 

 imotive force is the given magnitude, less heat is developed in a 

 bad conductor. The lightning problem is neither of these, but it 

 ;has quite as much relationship to the second as to the first. There 

 as a given store of energy to be got rid of, and accordingly the 

 heat ultimately generated is a fixed quantity. But the rise of 

 temperature caused by that heat will be less in proportion as the 

 production of it is slow; and though by sudden discharge a quan- 

 tity of the energy can be made to take the radiant form, and 

 -spread itself a great distance before final conversion into heat, 

 instead of concentrating itself on the conductor, yet this cannot 



be thought an advantage, for, just as in the old days a lightning- 

 rod was expected to protect the neighborhood at its own expense 

 by conveying the whole of a given charge to earth, so now it 

 must be expected to concentrate energy as far as possible on itself, 

 and reduce it to a quiet thermal form at once, instead of, by defect 

 of resistance and over-violent radiation, insisting on every other 

 metallic mass in its neighborhood taking part in the dissipation of 

 energy. 



The fact that an iron wire, such as No. 5 or even No. 8 B. W. G., 

 is electrically sufBcient for all ordinary flashes, and that resistance 

 is not a thing to be objected to, renders a reasonable amount of 

 protection for a dwelling-house much cheaper than it was when a 

 half-inch copper rod or tape was thought necessary. 



A recognition of all the dangers to which a struck neighbor- 

 hood is liable, doubtless prevents our feeling of confidence from 

 being absolute in any simple system of dwelling-house protection; 

 but at the same time an amount of protection superior to what has 

 been in reality supplied in the past is attainable now at a far less 

 outlay, while for an expenditure comparable in amount to that at 

 present bestowed, but quite otherwise distributed, a very adequate 

 system of conductors can be erected. 



Only one difficulty do I see. In coal-burning towns galvanized 

 iron wire is, I fear, not very durable, and renewal expenditure is 

 always unpleasant. It is quite possible that some alloy or coating 

 able to avoid this objection will be forthcoming, now that invent- 

 ors may know that the problem is a chemical one, and that high 

 conductivity is unnecessary. 



NOTES AND NEWS. 



The seventh annual meeting of the Association of Official 

 Agricultural Chemists, by a vote of a majority of the executive 

 committee, is called to meet in Washington, in the lecture-room 

 of the National Museum, at 10 a.m., on the 38th of August 

 proximo. 



— Professor E. 8. Woodward, for many years chief geographer 

 of the United States Geological Survey, has been appointed as- 

 sistant in the Coast and Geodetic Survey. Professor Woodward 

 was for ten years assistant engineer on the United States Lake 

 Survey, and was assistant astronomer of the United States Transit 

 of Venus Commission previous to his connection with the Geologi- 

 cal Survey. He was chairman of the Section of Mathematics 

 and Astronomy of the American Association for the Advance- 

 ment of Science in 1889, and is widely known for bis investiga- 

 tions in mathematics, astronomy, and physics. His appointment 

 to the Coast and Geodetic Survey is a subject for congratulation 

 on both sides. 



— Records have been received, at the office of the United States 

 Coast and Geodetic Survey, of observations made during the last 

 cruise of the " Pensacola.'" The stations include the West Coast 

 of Africa, and some islands in the North and South Atlantic. The 

 work was done by an officer of the survey. Assistant E. D. 

 Preston, aided by members of the ship's Oompaay. Gravity and 

 magnetic measures were made at St. Paul de Loanda (Angola), 

 Cape of Good Hope, St. Helena, Ascension, Barbadoes, and 

 Bermuda. In addition, magnetic observations alone were made 

 at the Azores (Fayal), Cape Verde Islands (Porto Grande), Sierra 

 Leone (Freetown), Gold Coast (Elmina), and in Angola at Cabiri. 

 The pendulums used in the gravity work were the ones employed 

 in 1883 in Polynesia, and in 1887 at the summit of Haleakala and 

 other stations in the Hawaiian Islands. The computations are 

 now under way at the office in Washington. 



— Mr. Ward McAllister called at the office of the Cassell Pub- 

 lishing Company, New York, the day before he left New York for 

 his farm at Newport, and delivered the manuscript of his book, 

 " Society as I have found it," into the hands of the president of 

 the company. Since he decided'to write the book, Mr. McAllister 

 has worked on it every day, and only completed it in time to leave 

 town before the Fourth of July. A glance at the manuscript 

 shows that it will more than fulfil the expectations of the public. 

 No more interesting volume of its kind has been written since 



