184 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1937 



ELECTRICAL FIELD RECORDS 



Measurements of the electrical fields of thunderstorms have added 

 considerably to our understanding of the operation of the thunder- 

 cloud as an electrical generator. Several investigators ^ ^ '^ "~^^ have 

 obtained records by different methods in the United States, England, 

 India, South Africa, Japan, and Sweden, all showing the same general 

 type of generation curve, A typical record obtained by Wilson ® is 

 shown in figure 3. This shows the regeneration of the electrical field 

 between a cloud and the earth after a stroke. The curve reminds 

 one of the charging curve of a condenser. In other words, the re- 

 generation of charge is much more rapid at the beginning and decreases 

 with increase in the amount of electricity accumulated in the charged 

 region. This is contrary to what would be expected if the separation 



CT -1000 H 



4-1 C> 



IE 



3° 



P +1000 -I 



Figure 3.- 



-Regeneration of the electrical field between a cloud and the earth after a lightning discharge. 

 (After Wilson.) 



of electricity by the air currents continued unhindered. In such a 

 case a straightline increase of charge, and therefore of electric field, 

 would bo expected. 



Wilson has advanced two reasons for the shape of the charging 

 curve. The first corresponds to the counter-electromotive force 

 principle which determines the shape of the charging curves of con- 

 densers. As the electrical field between the two charged regions of 

 the cloud increases, greater and greater opposition is offered to the 

 movement of the larger drops toward lower levels of the cloud and of 

 the smaller drops toward upper regions. This is obvious because the 

 negative charge in the lower regions of the cloud repels the large 

 negatively charged drops while the positive charge of the upper 



" Schonland and Craib, The electric fields of South African thunderstorms, Proc. Roy. Soc., ser. A, vol. 

 n4, pp. 229-243, 1927. 



'^ Halliday, E. C, Polarity of thunderclouds, Proc. Roy. Soc., ser. A, vol. 138, pp. 205-229, 1932. 



'» Wormell, T. W., Currents carried by point discharges beneath thunderclouds, Proc. Roy. Soc, ser. A, 

 vol. 115, p. 443, 1927; also Vertical electrical currents below thunderstorms and showers, Proc. Roy. Soc, 

 ser. A, vol. 127, pp. 567-590, 1930. 



w Norindcr, H., Electric thunderstorm field researches, Electrical World, vol. 83, pp. 223-226, Feb. 2, 1924. 



" Nukiyama and Nolo, Charges of thunderclouds, Japanese Joum. Astron. and Qeophys., vol. 6, pp. 

 71-81, 1928. 



