322 ELEMENTS OF ELECTRICITY AND MAGNETJSM. 



tricity until about 1896 when the electron theory had been de- 

 veloped. The present theory of atmospheric electricity as devel- 

 oped chiefly by Wilson, of Cambridge, England, is as follows : 

 When moist air is cooled, the water vapor always becomes super- 

 saturated unless there are nuclei present upon which the water 

 vapor can condense. It has been experimentally demonstrated 

 that both positive and negative ions can serve as condensation 

 nuclei, and that a lower degree of super-saturation is required to 

 cause the negative ions to act as condensation nuclei than is re- 

 quired to cause the positive ions to act as condensation nuclei. 

 The upper regions of the atmosphere where the ultra-violet rays of 

 the sun's light are very intense, are strongly ionized, and, when the 

 water vapor in these upper regions becomes super-saturated by 

 cooling, the negative ions, becoming loaded by the condensation 

 of moisture, fall towards the earth leaving the upper regions of 

 the atmosphere positively electrified. The great intensity of the 

 electric phenomena of the atmosphere during the summer time is 

 probably due to the fact that during the summer the condensa- 

 tion of moisture takes place at very great altitudes where the ioni- 

 zation of the atmosphere is very great, whereas during the winter 

 time most of the condensation which takes place occurs at very 

 much lower altitudes where the atmosphere is not strongly ionized. 

 Lightning protection. The use of the lightning arrester 

 for protecting electrical machinery is described in Chapter VI. 

 The use of the lightning rod for the protection of buildings 

 against damage by lightning is due to Benjamin Franklin. A 

 lightning rod is simply a good conductor leading as directly as 

 possible from a point above a building to a good ground connec- 

 tion in moist earth. A house which is not guarded by a lightning 

 rod may not be damaged, and, in many cases, houses which are 

 guarded, are severely damaged, but statistics show that the num- 

 ber of casualties is very greatly reduced by the use of lightning 

 rods. There is therefore no question as to the usefulness of the 

 lightning rod. Information concerning lightning rods may be 

 obtained from Sir Oliver Lodge's book Lightning Conductors and 

 Lightning Guards, Whitaker & Co., London, 1892. 



