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. [Illustration: Fig. 19.] [Illustration: Fig. 20.] [Illustration: 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.