LIGHTNING-RODS. 



LITERATURE, AMERICAN, IN 1875. 425 



diameter as the copper portion of the compound rod, 

 or equal to an iron rod two feet long and of the 

 same diameter as the iron portion of the compound 

 rod. By joining the rods end to end, the joint will 

 not be perfect. The iron rod being larger than the 

 copper one, the copper end will only be in contact 

 with one-seventh of the sectional area of the iron 

 end, and consequently a current, in passing from 

 the copper to the iron, will have to pass through a 

 sectional area of iron equal to the sectional area of 

 the copper end; or, in other words, it will have to 

 pass through a section of iron only one-seventh of 

 its entire section. This reduced section will offer 

 increased resistance. To make a perfect joint be- 

 tween them it would be necessary to increase the 

 size of the copper end in contact with the iron, so 

 that the copper would come in contact with the en- 

 tire surface of the iron end. Suppose we wish to 

 make a joint between a copper rod and the earth, 

 the copper rod having a sectional area of one inch. 

 If the joint is made perfect, there will be no greater 

 resistance to the passage of the current at the joint 

 than through any other portion of the copper rod. 

 For simplicity, consider the ratio of conductivity be- 

 tween earth and copper as 1 to 2, then it would be 

 necessary to have two square inches of earth-sur- 

 face in connection with the copper to make the joint 

 perfect ; but, instead of the ratio being as 1 to 2, it 

 is 1 to over 1,000,000,000 ; therefore it would require 

 over 1,000,000,000 square inches of earth in connec- 

 tion with the rod, or, in other words, the copper rod 

 would have to be connected to a metallic plate buried 

 in moist earth and having a surface area of 1,000,- 

 000,000 square inches, or a plate over 2,500 feet 

 square. These estimates are based upon low-ten- 

 sion currents ; for currents of high tension, such as 

 lightning, it would not require plates near so large. 

 As we have no means of testing the conductivity 

 of earth by using currents of such high tension, we 

 must leave it to the reader's imagination. In prac- 

 tice, it is impossible to obtain so great a surface by 

 burying plates in the earth, gas-pipes and water- 

 pipes being in contact with a system of large iron 

 pipes extending throughout a city ; the surface of 

 metal in contact with the. earth is great enough to 

 fulfill the conditions above, and consequently forms 

 a perfect joint. If we were to place a copper rod one 

 inch in diameter, having the usual defective earth- 

 connection, upon a building, it would not conduct 

 so great a quantity of electricity to earth as would a 

 wire the size of a silk-fibre having a gas or water 

 pipe connection. This can easily be proved by ex- 

 periment. It may be safely accepted as absolutely 

 certain that any building having rods such as ordi- 

 narily used, extending from its projective points to 

 the earth, and there connected with gas or water 

 pipes, also connecting with metallic roofs and spout- 

 ing, if there be any, is perfectly safe from the effects 

 of lightning. Where gas or water pipes are not 

 available, it becomes a very difficult matter to secure 

 good earth-connections. Coke being a good con- 

 ductor, it is a good plan to dig trenches several feet 

 long, leading from the building, and deep enough to 

 reach moist earth (dry earth not being a conductor), 

 and fill them with this material. Some earth should 

 be placed above the coke. The rod should be placed 

 upon the bottom of the trench. Coke and charcoal 

 retain moisture, but its principal advantage is in 

 its large surface- connection with the earth. When 

 charcoal is used it should be mixed with something 

 to improve its conductivity. Charcoal of itself is a 

 very poor conductor, but an excellent article to re- 

 tain moisture. The trenches should be dug near 

 the water-spouts, so they would be kept soaked 

 with the water passing from the roof. If wells or 

 ponds of water are available, the trenches or rods 

 should reach them. A very good rule would be to 

 connect the rods with every available piece of metal 

 in the vicinity that has any connection with the 

 ground. 



A recent examination of the lightning-con- 

 ductors of Paris, undertaken by R. F. Michel, 

 shows that rods may wear out, the joints be- 

 come imperfect, and the whole apparatus be- 

 come converted into an actual source of danger 

 to the building. Michel believes in a number 

 of sharp terminals for the upper end of the rod, 

 and in many cases he found these fused and 

 blunted, and in every instance where this had 

 occurred there was a defective ground-connec- 

 tion. His reasons for insisting on a number 

 of sharp terminal points are thus stated : 



Experience .teaches that, on the approach of an 

 electrified cloud, the more points there are the greater 

 (within a certain limit) will be the neutralizing effect. 

 When a stem, according to old usage, has but one 

 point, it acts only in one direction. But if there is 

 a large number of steins branching in all directions, 

 the preventive effect will be considerably increased. 

 This arrangement may be easily carried out by hav- 

 ing the ordinary conical-trunk copper arrow on the 

 top of the stem melted down and fashioned so that, 

 at about half its height, it will present a tolerably 

 distinct circular swelling. Into this swelling let 

 arrows be fixed, inclined at each side of the hori- 

 zontal plane to an angle of 45. 



By using six arrows or points for each inclination, 

 there will be twelve alternately in each direction. 

 Eadiating in all directions they will hasten the neu- 

 tralization of the electrified cloud ; and, in the event 

 of a discharge, the discharge, by dividing among 

 them, will prevent their fusion. 



On the subject of insulation he writes: 

 M. Perrot considering it necessary to isolate, with 

 great care, lightning-conductors from metallic mass- 

 es which form part of the building, recommends 

 the use of a glass insulating ring. Now, the ele- 

 mentary laws of physics prove clearly that, when a 

 lightning-conductor is in action, we may freely ap- 

 proach it and even touch it, since all the electric 

 fluid runs through it alone. There can, therefore, 

 be no inconvenience if we bind the conductor to 

 these metallic masses : there is found, on the con- 

 trary, a great advantage therefrom ; for these masses, 

 being in connection with the earth themselves, give 

 a certain amount of preventive effects. Besides, if 

 we assume, as an extreme case, that they are struck, 

 being in communication with the earth, the building 

 will be saved. 



His recommendations for forming ground- 

 connections are substantially in accord with 

 those of Mr. Miller, given above. In the for- 

 mation of joints he holds that it is not enough 

 to simply rivet the sections together with a 

 lap, or other style of adjustment, hut that in 

 every case the joint should be completely and 

 thoroughly covered with solder. The fewer 

 joints in the rod, according to this authority, 

 the better ; he therefore recommends the use 

 of galvanized iron cables, as these may be ob- 

 tained in greater lengths than iron in other 

 forms, thus doing away with a large number 

 of joints. 



LITERATURE, AMERICAN, AND LIT- 

 ERARY PROGRESS IN 1875. The last year 

 was a period of continued business depression, 

 and was marked by a diminished production 

 of books. The reduction in quantity was not 

 made up by improvemeBtt in quality. In the 

 higher walks of literature little was visibly ac- 

 complished. Our veterans plead no exemp- 



