Page:Popular Science Monthly Volume 25.djvu/842

824 In the circumstances which have been described in speaking of the accident there, a yard-square earth-plate could not have been depended upon to prevent the mischief. The lightning would still have preferred the largely developed root of the gas-mains to any such puny substitute, although such an earth-plate, well bedded in moist ground, might have served all purposes in the absence of so formidable a competitor. The condition of safety is that which has been so well stated by Professor Rousseau. The communication of the conductor with the earth must not be inferior to that of any neighboring mass of metal. When the arrangement for the earth connection has been efficiently settled, the conductor may be carried up from it, and this may with equal assurance be done either upon the single-rod system of Gay-Lussac or upon the multiple-rod principle of Professor Melsens, so long as the building is of moderate size and of a compact form. But, if the building is of large dimensions and of irregular form, the single conductor would of necessity have to assume an approximation to the multiple type, as the main stem is branched out above to bring every gable and turret and pinnacle of the structure under its protection. It is only when it has been completed by a broadly cast net of metallic meshes and lines that the old early dogma of the protected area can be now allowed to survive even in the mind of the engineer. When the work of construction has been so far carried out it is still, however, not to be looked upon as complete until the stamp of efficiency has been placed upon it by the application of the final test, which the advance of electrical science has now placed in the hands of the constructor. It is the crowning distinction of this system of defense that by a very easy process it can be at once ascertained whether all the arrangements of the engineer have been properly carried out. By the employment of the ingenious piece of apparatus which is known as the "Differential Galvanometer," the electrician can in a few minutes ascertain what the resistance is that would be offered between the air-terminal and the earth communication of a conductor, if a discharge of lightning fell upon the rod. That resistance must never be left unheeded if it amounts to anything in excess of the quantity which is technically known as two ohms. It is quite possible, indeed, by the exercise of judgment and skill, to reduce the resistance in every case somewhat below that. With a conductor which has recently been erected upon the Hall of General Assembly in Edinburgh, it was found at the final test that the earth resistance was only the 0·7 of an ohm. But the galvanometer test must not only be applied as the last step of the construction; it must also be drawn upon from time to time, and at not too distant intervals, to ascertain how far the originally well-conceived and well-executed work is or is not in process of being injuriously affected by the physical agencies that are at all times in antagonistic operation to the constructive efforts of man. The free and frequent use of the testing galvanometer is, indeed, the