The system employed by Mr. R. S. Newall, F.R.S., for the construction and erection of lightning conductors is probably the most complete—and certainly the most representative—of the various methods in vogue in England. The special study Mr. Newall has made of the subject in all its bearings, both theoretical and practical, added to the fact of his possessing at his extensive cable works at Gateshead such exceptional facilities for the production of copper ropes and bands composed of the purest metal, render him one of the first authorities on all matters connected with the application of lightning conductors to buildings. In describing, therefore, the English method, reference will chiefly be made to this gentleman’s apparatus and inventions.
The function of a lightning conductor is twofold. In the first instance, it operates as a medium by which explosions of lightning, or, to speak more accurately, disruptive discharges of electricity, are led to the earth freely, and without the risk of their acting with mechanical force, as they invariably do when compelled to pass on their way to the earth through so-called non-conductors, that is to say, bodies possessed of low conductivity, such as the atmosphere, wood, stone, &c. In the second instance, the conductor acts as a means whereby the accumulation of electricity existing in the atmosphere is quietly drawn off and carried noiselessly into the earth, and dissipated in the subterraneous sheet of water beneath it. Now this accumulation of electricity, always greatly intensified during a thunderstorm, invariably seeks the easiest road to earth; this road is technically called ‘the line of least resistance.’ This line of least resistance is influenced by various circumstances; the resistance of any line may be lessened by the presence of streams of warm vapour or rarefied air such as would come from chimneys, from barns or stacks containing new hay; by a column of smoke, or by the presence of tall trees moist from rain. It is not always easy to find the reason why the lightning takes any particular path, but one thing is certain, that is, it acts under certain fixed principles, and does not take any particular route by chance, but always because it is the line of least resistance. What the lightning conductor really does is to prevent the possibility of an electric discharge within a certain district, for instance, in the interior of a house or other building.
From the above remarks, it will easily be seen that lightning conductors should be made of materials possessing the highest possible power of conductivity, and be large enough to carry off the heaviest electric discharge that is ever likely to fall upon them. The various metals being by far the best conductors of electricity, it follows that the lightning conductor must be constructed of metal of some kind. But even metals differ to a great extent in their conducting powers, as has been shown in a previous chapter. There are, however, only two metals which are practically available for use as lightning conductors, namely, iron and copper, and after repeated experiments Mr. E. S. Newall has arrived at the conclusion that a conductor made of copper of adequate size is the best—and, in the end, the cheapest—means of protecting buildings from the effects of lightning. The relative conductivity of iron and pure copper being as six to one, it follows that if a copper cable or bar of a given size be sufficient, an iron cable or bar ought to weigh six times as much per lineal foot in order to be equally safe. It may be added, that while copper is more expensive, weight for weight, than iron, it is not so liable to oxidise; nor, on account of its higher conducting power, is it so easily fused. The comparative smallness of its mass renders it far more manageable than iron, and does not interfere with the architectural features of the building on which it is used. On the contrary, it is readily adapted to curves and angles.
It may therefore be taken for granted that, almost without exception, pure copper is the best material that can be used in the construction of lightning conductors.
Fig. 19. Fig. 20. Fig. 21.
The size of the terminal rod or point used in Mr. Newall’s method varies in length and diameter according to the extent and height of the building to be protected. As a rule, they are from three to five feet in length, and from five-eighths to three-quarters of an inch in diameter; at the upper end they branch out as shown in [fig. 19].
In conjunction with this terminal rod a short description of the ‘Auffangstange,’ or ‘reception rod’ of the Germans, may be given. This ‘reception rod’ (see [fig. 21]) is made of iron, and varies in length from ten to thirty feet. It consists of two parts, the higher part, which measures two-thirds of the whole length, is fastened by a flange to the lower part of the rod. In fixing this German ‘reception rod,’ its height and weight have to be taken into consideration. It is generally made fast by two strong staples, b and c, as shown in [fig. 20], which pass through the king post of the roof and are fastened behind by screw-nuts. The part marked d rests in the lower ring c so that it cannot sink, and the extreme end passes through this ring c and is screwed tightly to the nut e; f is a cap to prevent the rain getting into the roof.
It is much to be regretted that not only professors and amateurs studying the manifestations of the electric force, but even learned societies, such as the French ‘Académie des Sciences,’ should have spread so many imaginative theories about this ‘reception rod.’ At the bottom of all was the fancy, not often declared, but still visible in its expression, of the metallic conductor possessing some occult power of attracting lightning. In France, as well as in Germany and Italy, there existed for a long time, and to some extent still exists, quite a mania for erecting huge rods, such as that shown in the engraving (see [fig. 21]), on the top of buildings, the general belief being that the more high-towering the greater would be the ‘area of protection.’ A little common sense, brought to the aid of fanciful imaginings, should have taught the supporters of this ‘area-of-protection’ theory that it was absolutely untenable. The electric force, seeking its nearest path to the earth, could not be expected to diverge from it through the action of a rod raised somewhat higher than the surrounding building; and the proper method clearly was to bring the metal everywhere as near to any possible emanation of the force, whether lateral or vertical, as could be done. Besides being really of no use, except in rare instances, such as the neighbourhood of high trees, these tall rods formerly employed, and still frequently seen on the roofs of buildings, had the detriment of being unsightly, while at times they were positively dangerous. Instances occurred in which a high wind threw them down from their elevated position into the road below, on the heads of passers-by. Thus two persons were killed in Paris in the summer of 1830 by the fall of a gigantic ‘tige’ from the steeple of the church of St. Gervais. Either at the same moment, or immediately before, a stroke of lightning fell upon the church in its lower part, away from the conductor, making a hole in one of the walls, and then escaping, without doing further damage, by some iron water-pipes running underground. The conductor in this case had been constructed on the model approved by the ‘Académie des Sciences,’ but the accident conclusively showed that there was no trust to be placed in any mere theoretical calculations as to the extent of the ‘area of protection.’
A noteworthy example of the fallacy of the ‘area-of-protection’ theory is to be found in the case of the explosion at the powder magazine at the Victoria Colliery, BurntclifFe, Yorkshire, which was struck by lightning and destroyed on August 6, 1878. The instance is also instructive as showing how important it is that copper conductors should possess the highest possible conductivity—i.e. be made of the best and purest copper.