536 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK v. 



II. DESCRIPTION AND ARRANGEMENT OF LIGHTNING-CONDUCTORS. 



A lightning-conductor is nothing else than a good conductor 

 without interruption, the upper end of which is raised to a sufficient 

 height to command the edifice it has to protect, and the lower end 

 communicates freely with a subterranean water-bearing stratum. 



As lightning can melt and volatilize metallic threads of a small 

 diameter (less than six millimetres), but has never be'en known 

 to bring even to a low red heat, square rods of iron fifteen 

 millimetres in the side, the conductors should be made of not less 

 dimensions than this. 



The lightning-conductor is composed of two principal parts, the 

 rod and the conductor or conductors; the description of which is 

 now given. 



The iron rod which forms the upper end should be terminated by 

 a cylinder of red copper of 2 centimetres diameter and of 20 to 25 centi- 

 metres in length, fixed by a screw to the rod. This cylinder is itself 

 terminated at the top by a cone. The lower part of the rod is square, 

 and gradually increases in thickness to the point of junction with the 

 conductor, where the section measures about 4 or 5 centimetres on the 

 side. In this case the total height of the rod varies from 3 to 5 metres. 

 Formerly it was recommended to terminate the rod by a fine and 

 very sharp point of gold or platinum. When this was done as soon 

 as the storm commenced the electricity passed away through the 

 point in the form of a luminous brush visible in the dark. The highly 

 electrified air in passing to the cloud neutralized, it was thought, a 

 portion of the electricity of the latter. But the intensity of the 

 electric flow was sufficient at the same time to melt the gold or pla- 

 tinum point, so that after a certain time the sharp point disappeared 

 and was replaced by a large button of fused metal. 



The preventive action of the sharp point in drawing off the elec- 

 tricity in the form of a kmiinous brush, was only assured for a limited 

 time ; besides this, it was not a very great advantage, if it were true 

 that the air electrified by the rod instead of going directly to the cloud 

 was often driven away laterally by the wind. For these reasons the pre- 

 ference is now given to rods ending in a cylinder and cone of copper. 

 Formed in this way the point of a lightning-conductor very seldom 



