626 peters: protection against lightning 



2. During each year there are approximately 1500 persons 

 affected by lightning stroke in the United States, one-third of 

 this number being killed and the rest subjected to injuries which 

 in many cases are permanent. About nine-tenths of these acci- 

 dents occur in rural districts. 



3. Such evidence as is available on the effectiveness of light- 

 ning rods indicates that, taking rods as they come in the gen- 

 eral run of installations, they reduce the fire hazard from light- 

 ning by 80 to 90 per cent in the case of houses, and by as much 

 as 99 per cent in the case of barns. The same is undoubtedly 

 true of other buildings having characteristics similar to those 

 of barns and houses. 



4. With regard to the proper metal for Ughtning rods it may 

 be said that the differences of resistivity of the metals ordinarily 

 available for lightning rodfe are not great enough to make one 

 metal preferable to another. Resistance to atmospheric and 

 soil corrosion is the chief essential to be considered. 



5. Extended metallic masses on or within a building must 

 be made a part of the lightning rod system, with a possible excep- 

 tion, however, in the case of metallic masses within the building 

 which do not come near the roof and are at a distance of ten 

 feet or so from the rods. Gas pipes should be avoided in erect- 

 ing lightning conductors if possible, but if they are so located 

 that it is impossible to keep at a distance of ten feet or more, 

 they should be electrically connected to the rods at several 

 points, connected to earth within the building, and well bonded 

 around the gas meter. 



6. The maximum current in a lightning flash may, in some 

 cases, be more than 20,000 amperes. 



7. Each flash of lightning consists, in most cases at least, of 

 a number of consecutive discharges along the same path with 

 short time intervals between them. The duration of each of 

 the consecutive discharges is of the order of 1/35000 second. 

 The best obtainable evidence points to the fact that these con- 

 secutive discharges are unidirectional and of steep wave front. 



8. The heating effects of a lightning stroke on a rod of ordi- 

 nary size, i.e., a rod weighing about 0.5 kg. per meter, is not 



