In the case of silken garments the evidence met with does not warrant a statement either for or against them; yet there appears to be no reason why this non-conductor should be more of a safeguard than any other. No doubt an abundance of gold and silver lace, or cloth having threads of these metals, might prove a protection. Feather beds, too, have been regarded as places of safety, but persons have been killed by lightning while in bed. Dr. Franklin advised especially that the vicinity of chimneys be avoided, because lightning often enters a room by them. All metallic bodies, mirrors and gilded ornaments, he held, should likewise be shunned. Contact with the walls or the floor or proximity to a chandelier, a projecting gas-pipe, a position between two considerable pieces or surfaces of metals, unless distant, are all hazardous. Draughts of air are also to be avoided. Bell-wires may generally be considered as protective, though too small to be effectual. Perhaps a hammock, in addition to the preceding precautions, will afford as much security as can be derived from insulation. But in a building having continuous iron walls, posts or pillars from top to bottom, or in one which is properly supplied with conductors in other forms, all the foregoing precautions may be neglected without apprehension. Yet, as was suggested early in this article, the great number of buildings damaged by lightning while furnished with rods has caused much distrust of this system of protection.
From the large number of trees receiving the electric current it has come to be thought by many that these may be the best protectors of buildings if properly placed. In a case coming under my observation a tree received (or at least deflected) the current and communicated it to the house. In many instances, however, the building is struck while tall trees near by are untouched.
There is no doubt that lightning generally strikes elevated rather than low objects, and therefore it has been thought that a building surrounded by steeples had nothing to fear. As previously stated, however, the bolt sometimes selects a low object when high ones are at hand. For example, lightning fell upon a house occupied by Lord Tilney in Naples, although it was surrounded on all sides, at the distance of four or five hundred paces, by the towers and domes of a great number of churches, all wet with a heavy rain.
In considering the matter of protection from lightning we must bear in mind that trees, buildings, masts and other elevated points exert no attractive power on the thundercloud except in connection with the great plane where they are situated. The primary cause of the discharge is not in the metals of the building, the exact point or line in which the insulation by the air breaks down being determined by a variety of causes. The elevated points of a building or ship may form a channel for the passage of the current, but it is not the only one nor the cause of the discharge, which would take place sooner or later though the ship or building were absent altogether.
There has been a difference of opinion in regard to the area protected by lightning-conductors, early notions on this point having been much exaggerated. Leroy's, in 1788, is the earliest positive statement which I have met. It is, that a conductor protects a horizontal space around it equal to somewhat more than three times the height of the metal rod above the building to which it is attached. The physical section of the Academy of Sciences of Paris, on being consulted by the Minister of War in 1823, expressed the opinion that a lightning-conductor protects a circular space of which the radius is equal to the height of the rod. Here, apparently, is a wide difference, but possibly the estimates refer to different elevations. Leroy clearly intended an area at a level with the top of the building: thus, supposing the rod to be attached to a chimney six feet in height and to rise a foot and a half above the chimney, then it would protect a radius of about ten feet on the roof. The estimate of the Academy of Sciences speaks of the total height of the rod, and refers to a horizontal area at the base of the measurement, whether this began at the ground or at the top of the structure to which the rod is attached. In this view the estimates do not differ so much as might appear, the latter being about one-third less than that of Leroy. Other French writers estimate the area protected as having a radius of double the height of the rod above the highest point of the connected structure, being twice the radius allowed by the Academy. Later physicists have been cautious in giving figures, for experience has shown that estimates of protection are not accurately observed by the descending bolt. For instance, when Her Majesty's corvette Dido, furnished with the best system of conductors, was struck by lightning, the discharge fell in a double or forked current upon the main royal mast, one of the branches striking the extreme point of the royal yard arm and passing along to the conductor on the mast, while the other fork fell on the vane, spindle and truck; which last was split open. As soon as the discharge reached the conductor all damage ceased.
The practice of the best electricians has now long been to protect all angles and projections, the latter by a branch of the rod, and the former by running a line of rod over them, having at every few feet sharp points of an inch or two in length attached to and standing out at right angles with the rod. Indeed, some go even beyond this, forming points along the whole length of the conductors by notching the corners of a square rod with a chisel. Sometimes a rod is twisted for ornament, but with a loss for practical uses, for in a twisted rod the electrical current is retarded, and a portion of the charge is more liable to leave the conductor.
In England, during our Revolutionary war, an active scientific discussion was carried on as to whether the upper end of a lightning-conductor should be sharp or blunt. "The scientific aspect of the question soon became lost in political acrimony, those who, with Dr. Franklin, advocated sharp conductors, being classed with him and the Revolutionary party, while those who advocated blunt conductors were held to be loyal subjects and good citizens." There is a difference in the action of a sharp conductor and one with a blunt end or terminating in a ball. In the first the point silently receives the current, while in the other the opposite electricities of the rod and cloud may meet with explosion; but the building will not necessarily be injured from this cause. M. Michel proposed to combine the advantages of the two systems by having the rod terminate in a spherical enlargement from which should project points in various directions. This, he thought, would lessen the danger of fusion and control the current at distances where it might escape other forms of terminal. Some American electricians now use a modification of this form, surmounting the rod with a branching tip, while others prefer the single point. The latter is the form used in the American and British navies. The vane, with its appurtenances, is sometimes made the terminal of the conductor, and should at least always be connected.
The practice is also a good one of combining balustrades, finials and other metal-work at the tops of buildings with the system, by which protection is rendered more complete. Especially is it important to connect with a metallic roof at its lower edge, and with the gutters, unless the rain-conductors connect with the earth to its moist mass.
In regard to the material for conductors, copper is undoubtedly the best, but more expensive than iron. The latter is more liable to rust, and on account of its lower conductive power is more easily melted. An electrical explosion which only melts a copper wire would utterly destroy an iron wire of twice the diameter of the former. In being heated a rod contracts in length, and is then liable to fracture by the shrinkage, but if of sufficient size these results are not likely to occur. An iron rod, by successively receiving an electrical discharge, is sometimes reduced in size.
The conducting power of metals likely to be found in buildings is as follows: taking the power of lead as one, that of tin will be two—that is, tin conducts electricity twice as well as lead; iron, nearly two and a half times as well; zinc, four times; and copper, twelve times. From this comparison of conducting power the important fact will appear that when any two of these metals are used in the same line of conduction, the one of low power should be proportionately larger. Sir W.S. Harris—perhaps the best authority on lightning-rods in general—advises that the size of the rod, if of iron, should be three-fourths of an inch in diameter, although he admits that probably never in the experience of man has a rod half an inch in diameter been melted by an electrical discharge. He regarded the extent of surface rather than quantity of metal in the conductor as the measure of its power, while many other electricians hold the contrary opinion.