862 



SCIENCE 



[N. S. Vol. XXX. No. 781 



frequeBcy and severity of thunder-storms 

 in our lower air accompanies the active 

 period of the sun or sunspot maximum. 

 This is a hypothesis which would require 

 a careful collection and comparison of 

 data over a long period to give it status as 

 a scientific fact or wholly to disprove it. 

 Be that as it may, experience with lightning 

 damage in electric installations seemingly 

 supports the idea, and led me in a paper 

 given some seven or eight years ago during 

 the minimum period, to predict a severe 

 ordeal a few years in advance. As a mat- 

 ter of fact the prediction was to a large 

 extent verified with the result of extraor- 

 dinary activity in devising safeguards 

 from which the electrical engineering art 

 now benefits. In general the harm done 

 by thunderstorms is due directly or in- 

 directly to the heavy spark discharges 

 called lightning flashes or strokes of light- 

 ning. 



It may be of interest to refer briefly to 

 the conditions existing in a cloud which is 

 the source of such destructive energy. As, 

 is well known, clouds consist of fine water 

 particles suspended in the air. When 

 frozen these particles are crystalline like 

 minute snow crystals. All clouds above 

 the snow line are likely to be of that char- 

 acter. At a temperature above freezing 

 the particles of water are microscopic 

 spheroids which may by gradual coales- 

 cence form drops of rain. This process of 

 coalescence necessarily diminishes the total 

 surface of the water existing as such in 

 the cloud. Should, however, the original 

 particles possess even a slight electric 

 charge, the union of the drops, by lessen- 

 ing the total surface, or diminishing the 

 electric capacity, results in a great rise of 

 potential or electric pressure on the sur- 

 face of the drops. The process of coales- 

 cence continues and the water falls out of 

 the cloud as rain. If the cloud particles 



are frozen the diminution of surface and 

 consequent increase of electric pressure 

 can not take place. This would seem 

 sufficient to account for the general ab- 

 sence of thunder-storms in winter, though 

 perhaps other causes contribute. 



A thunder-cloud has been compared to 

 an insulated charged condiictor, such as a 

 body of metal hung upon a silk cord, but 

 in reality the two are not at all com- 

 parable. It is a mistake to assume any 

 close analogy to exist. The cloud being 

 only an air body containing suspended 

 water particles, is not a conductor, nor can 

 it, as in the case of metal, permit the ac- 

 cumulation of its electric charge on its 

 outer surface. In fact it possesses no true 

 definite outer surface but blends with the 

 clear air around it. The electric charge it 

 possesses remains disseminated, so to 

 speak, throughout, and must reside chiefly 

 upon the surface of its constituent water 

 drops. Accumulation in any part would 

 require the insulating air between the 

 drops to be overcome. 



A lightning stroke from such a mass 

 may indeed represent a discharge of hun- 

 dreds of amperes at millions of volts. We 

 must, however, be cautious not to exag- 

 gerate either the current or the potential 

 present in a lightning flash. The current 

 in a flash can at times be only a few am- 

 peres or may in the heavier discharge 

 reach perhaps hundreds, or possibly in 

 extreme cases some few thousands of am- 

 peres. It is doubtful if the potential much 

 exceeds at any time more than a few mil- 

 lions of volts as it is probable that small 

 local breakdowns start the disruptive proc- 

 ess which then extends through miles of 

 length. The individual water particles 

 even when collected into drops can not be 

 charged to such enormous potentials as 

 millions of volts. In reality it is the com- 

 bined effect of the ntunerous particles act- 



