22 



The Cause of Thuot)ee storms. 



Atmospheric electricity has been the subject of considerable study. In earlier 

 days friction of the air against the earth and objects on it, and also evaporation, 

 have been mentioned as possible causes of atmospheric electrification, but during 

 the last few years another explanation has been advanced. It has been proven that 

 the earth is always negatively charged, and that the air is a conductor, although a 

 poor one, to be sure. Since a charge cannot reside within a conductor it follows 

 that the charge on the surface of the earth must be transferred more or less quickly 

 to the outside of the conducting atmosphere. Within the last four or five years 

 the rate at which the earth loses its charge to the air has been measured by Dr. G- C. 

 Simpson, of Simla, India, and although the rate of loss is small per square foot 

 or per square yard, yet when the entire surface of the globe is considered it amounts 

 to a constant current of more than 1,000 amperes! Despite this continual loss, 

 the earth's charge is maintained at a uniform potential, consequently it follows 

 that the earth must be receiving an amount of negative electricity equal to that 

 lost. The idea at once occurs that the rain brings back the required negative charge. 

 However, careful measurements, at half-a-dozen different places distributed over 

 three continents, show that in all kinds of rain more positive than negative elec- 

 tricity is brought to the earth! This leaves the source of renewal of the earth's 

 charge still unknown, and, moreover, offers no explanation of the existence of 

 positive charges in the air, so that the difficulty surrounding the problem of atmo- 

 spheric electricity has been increased rather than diminished. 



We do know, however, that under certain conditions, when water vapour high 

 up in the atmosphere condenses into minute drops these become charged with elec- 

 tricity. ^Tien one of these drops is formed from several smaller ones there is 

 less surface for the electricity to be spread over, and consequently it becomes more 

 dense. Thus it happens, that in the cloud there are drops of various sizes, some 

 of which are more strongly charged than others. The weak charges appear nega- 

 tive to the stronger, and consequently the electricity has the effect of attracting 

 the drops together more rapidly than they otherwise would unite. This accounts 

 for the ver^^ sudden development of thunderstorms. When the condensation has 

 gone on sufficiently the cloud becomes one great conductor, and the electricity leaves 

 the surface of the drops and collects on the outside of the cloud. This produces 

 another increase in intensity of the charge. Then, again, the cloud acts as one 

 plate of a condenser, the earth as the other, and with the air between they actually 

 form a great condenser similar in principle to the little plate condenser shown in 

 Figure 13. Thus it is that such immense charges of electricity are produced in the 

 cloud. As the positive gathers together in the cloud, the negative in the earth 

 collects directly beneath, and the attraction of these unlike charges draws the cloud 

 downward, so that all things are favourable to a violent flash once the electric 

 pressure or attraction between the cloud and earth is sufficiently high to make the 

 electricity jump the decreasing space between them. 



Nature of Lightning Discharge. 



But the lightning discharge is not a single rush in one direction. A steel 

 spring or a violin string drawn aside and let go will fly away past the centre, then 

 back, and continue to oscillate a large number of times, the oscillations gradually 

 dying out as the energy is dissipated. That is Just what happens in the lightning 

 stroke. When the current breaks through, the discharging process goes too far, 



