peters: protection against lightning 627 



likely to be appreciable except at the place where the stroke 

 enters the rod, or at high resistance joints. 



9. Good mechanical construction in a lightning rod system 

 is a prime essential to permanency. Rods are subject to severe 

 strains from wind, snow and ice, thermal expansion and con- 

 traction, and, in the event of a stroke of lightning, to electro- 

 magnetic stresses, so joints must be strong and the rod securely 

 fastened to the building. 



10. The resistance of the earth connection should be made 

 as low as practicable. In practice there is no chance of getting 

 too low a resistance. On the other hand, a resistance which 

 rises above 15 or 20 ohms at any time should be considered as 

 excessively large on account of the potential drop which may 

 possibly be set up in the event of a stroke. 



11. When a system of hghtning rods is installed aerial termi- 

 nals with points should be placed at all chimneys, gables, points 

 or other projections toward which a stroke of lightning might 

 be directed. This is necessary because a point cannot be relied 

 upon to protect objects other than that upon which it is placed. 



12. Down conductors should be run in such a way that a 

 stroke on any aerial terminal on a structure will have two or 

 more widely separated paths from the foot of the aerial terminal 

 to earth. One path to earth has been found to be unsafe, and 

 more than two are preferable. 



13. It has been shown photographically that the path of a 

 lightning discharge may be shifted by the wind as much as 10 

 meters or more during the period between the initial and final 

 discharge. For this reason it is advisable not to allow too great 

 an expanse of flat roof to be exposed without aerial terminals. 



14. The return on an investment in lightning rods may be 

 expected in two ways; in a sense of personal security from 

 hghtning, and in actual security to life and property'. The 

 property loss from lightning is not sufficient to cause universal 

 protection against lightning to be a paying investment. Pro- 

 tection against lightning is justified as an investment only 

 where risk to human life is involved, or where the property risk 

 is great enough to make protection against lightning more eco- 

 nomical than insurance. 



