Page:The American Cyclopædia (1879) Volume X.djvu/471

 LIGHTNING 465 state, the stratum of air between acting like the insulating glass plate between the two me- tallic surfaces ; and when at last the attraction between the two opposite electricities becomes too strong for the interposed medium to resist, they rush together, producing the disruptive discharge accompanied with the flash and re- port. With a good conductor passing from the cloud to the earth, the electrical equilibrium would be silently restored, as a Leyden jar is quietly discharged by connecting its inner and outer surfaces with a wire pointed at each end. But if an imperfect conductor is interposed, the electricity, seeking to follow this, may pro- duce the most violent effects, and these are exhibited at the points where the continuity of the conductor is imperfect or interrupted. This is well illustrated in the common experi- ment with the model of a house loosely put together and furnished with an interrupted rod, through which an electrical shock is con- veyed. The effect is to throw the model into pieces ; but when the same experiment is tried upon a complete rod, the discharge takes place without violent action. Sir W. Snow Harris also illustrates the effect of an interrupted con- ductor by scattering bits of gold leaf upon pa- per, and passing along them an electrical dis- charge, sufficient to burn the gold and blacken the paper. But it is observed in this experi- ment that only those bits are burned, and the portions of them only, which lie along the line of most perfect conduction or of least resis- tance ; the paper too will be nowhere blackened except on this line. Similar phenomena are observed upon a large scale in almost every instance of a house being struck by lightning. The path of the electrical current is traced along the best conductors, and as the lightning passes from one to another the most destruc- tive effects are observed in these breaks. Im- perfect conductors lying near are shattered to pieces or scattered about, and the effects of in- tense heat are developed where the current is most obstructed. The animal system offering a good conductor, the lightning leaves more imperfect ones to pass by this on its course, and thus men and beasts are frequently struck when standing near projecting objects, as trees, that present themselves as convenient mediums for the reestablishment of the electrical equilibri- um. Franklin, having satisfied himself of the identity of lightning and electricity, was not long in drawing from his discovery practical results of immense importance in protecting buildings from the stroke of lightning ; and he thus announced in his "Poor Richard's Alma- nac" for 1753 his invention of the lightning rod, the description being nearly as complete and exact in all its essential particulars as could now be given after the experience and trials of more than a century: ''How to Secure Houses, <&c., from Lightning. It has pleased God, in his goodness to mankind, at length to discover to them the means of securing their habitations and other buildings from mischief by thunder and lightning. The method is this : Provide a small iron rod (it may be made of the rod iron used by the nailers), but of such a length that one end being 3 or 4 ft. in the moist ground, the other may be 6 or 8 ft. above the highest part of the building. To the up- per end of the rod fasten about a foot of brass wire, the size of a common knitting needle, sharpened to a fine point ; the rod may be se- cured to the house by a few small staples. If the house or barn be long, there may be a rod and point at each end and a middling wire along the ridge from one to the other. A house thus furnished will not be damaged by the light- ning, it being attracted by the points, and pass- ing through the metal into the ground without hurting anything. Vessels also having a sharp- pointed rod fixed on the top of their masts, with a wire from the foot of the rod reaching down round one of the shrouds to the water, will not be hurt by lightning." Thus Frank- lin merited the words of the French medal sub- sequently struck in his honor, Eripuit ccelo fulmen, though from a passage found among the fragments of Ctesias (PJiotii Biblioiheca}^ it would seem that some knowledge was pos- sessed by the ancients, 400 years before the Christian era, of the effect of iron rods in averting the lightning. The writer in this pas- sage makes mention of a fountain in India, from the bottom of which was obtained a kind of iron, which being set in the ground averted clouds, hail, and lightning. Various modifi- cations in the construction of the rod have since been proposed, and copper has been ad- vantageously substituted for iron, as in those planned by Sir W. Snow Harris for the use of the ships of the royal navy. These protectors are in bands of copper, overlapping each other so as to break joints, and are let in to the after side of each mast. They pass down to the keel, and are continued through this by copper bolts into the water ; they also connect with copper bands laid under the deck beams and continued through the side of the ship. Har- ris also made conductors for buildings of cop- per pipes firmly screwed together, and fur- nished at top with a pointed extremity ft. long and f in. in diameter. The tubes for a given amount of metal expose the greatest surface, and thus furnish the maximum ca- pacity of conduction of the electrical current. Copper moreover conveys the current more freely than iron in the proportion of 12 to 2. This is an important feature, inasmuch as, hav- ing no measure of the power of the current that may strike the rod, we should provide one of sufficient size for any stroke. An iron wire may be entirely inefficient, and melt beneath the electrical current, or this may be divided and bound off to other more or less perfect conduc- tors near the rod. It is this inefficiency or im- perfect construction of rods in use that has led many to question the value of any metallic con- ductors, and even to imagine that they all serve to attract lightning, and thus increase the dan-