LIGHTNING PEEK 197 



These data are useful in cletermining the best methods of pro- 

 tection, because they tell what will happen when conditions are such 

 that discharges can take place to the house or rod under con- 

 sideration. In other words, the data give the chance of an object 

 being struck when it is directly under a cloud at sufficient potential 

 to cause a discharge. To determine the chance of any object being 

 struck during a year, it is also necessary to know the chance of a 

 cloud of sufficiently high potential passing over the object. It is 

 apparent that this chance is usually quite small even when the 

 object is located on a plane. When mountains, hills, and trees are 

 added the chance becomes still less. 



PROTECTION OF BUILDING 



The lightning rod is of real value in preventing or limiting dam- 

 age when a building is struck. Whether or not it increases the 

 chance of being struck depends upon circumstances. It is rather 

 doubtful if it often does, as a chimney, grounded gutter, or water 

 pipe may be sufficiently conducting to determine the direction of the 

 stroke. 



In deciding upon a lightning rod the economic factor or the cost 

 of protection must be considered in connection with the probability 

 of being struck. It seems that the best results can be obtained with 

 plain inconspicuous rods. Ornate rods or rods with special points 

 appear to have no advantage. A grounded metal roof would seem 

 almost ideal protection. Rods should be installed so that side 

 flashes are not likely to occur to conducting objects inside or outside 

 of the building. When a building is in an exposed position and 

 storms frequently pass over it, a more elaborate outlay for protec- 

 tion would be justified than in the case of an unexposed building. 

 For instance, a building on a cliff over a valley followed by storms 

 would be in much greater danger from lightning than a building 

 located in the valley. This would be especially so if connections 

 were made from the upper building to water level. Lightning rods 

 can be arranged to give almost ideal protection when the expense is 

 warranted. In the case cited above, a lightning rod located on the 

 cliff would give good protection to buildings in the valley. It is 

 probable that except for buildings in exposed positions and for 

 special cases as magazines or oil tanks, the cost of even an inexpen- 

 sive rod is not warranted from the standpoint of the cost of in- 

 surance. 



SPARKS INSIDE OF METAL TANKS 



Tests have been made to determine if it is possible to cause sparks 

 inside of metal tanks or cans by direct stroke or by electromagnetic 



TG041— 20 14 



