Page:Popular Science Monthly Volume 25.djvu/837

Rh of insufficient dimensions, and of bad principles of construction, are by no means to be looked upon with tolerance, to say nothing of favor, notwithstanding the occasional good service that may be entered to their account.

Irrespective of all theoretical considerations, and upon purely experimental and demonstrative grounds, it is possible in the present state of electrical science to definitely state what it is that an electrical engineer has to do when he undertakes to protect buildings against the destructive force of lightning. He has, in the first place, to make sure that, wherever the lightning can fall, it shall find an open and practically unobstructed path to traverse in its passage to the ground. He is quite sure that the electric discharge will confine itself to the track of a conductor, and will pass quietly and harmlessly along it, provided its dimensions are adequate to the task of transmission, and provided the inlets and outlets are sufficiently capacious for its unimpeded reception and escape. It is a thoroughly established and altogether indisputable canon of electrical science that when a discharge has to pass through a conductor of too narrow size, and with obstructed inlets and outlets, it, of necessity, accomplishes its passage as a turbulent and ill-regulated force all the way, with a tendency at every step to make a devious outburst or overflow; and that when it passes through a conductor of ample dimensions, and with unimpeded ingress and egress, it is devoid of all erratic impulse, and traverses the appointed channel as an obedient and well-trained power. The task of the engineer, therefore, resolves itself primarily into so arranging his apparatus as to keep the lightning in its well-ordered and harmless state so long as it is in the close neighborhood of buildings that might be injured by any uncontrolled outburst through a devious path. There are three ways in which he can seek to accomplish this purpose. He can multiply and, as it were, enlarge the gates of ingress by increasing the number of his air-terminals and earth contacts through which the discharge may have to be gathered into the conductor. He can augment the dimensions and the carrying capacity of the conductor, and he can amplify the outlets of escape, whether in the direction of the cloud or earth. Where these conditions have been properly secured, there is not the most remote probability that the conductor will fail in its appointed task. This is not a question that is now open to doubt. It is as certain that the lightning will traverse a well-arranged and competent conductor, rather than the building to which this is attached, as it is that the electric spark from the charged conductor of an electrical machine will strike a brass ball and rod, and will not strike a stick of sealing-wax or of dry wood, when these are presented side by side. As a matter of fact it is sometimes imperfectly insulated tracts of the surface of the earth that are inductively charged by the propinquity of an overhanging storm-cloud, and sometimes the overhanging cloud that is inductively charged by disturbances originating in the