1. 1) workman, E. J, and S. E. Reynolds, 1950: Electrical phenomena occurring during the freezing of dilute aqueous solutions and their possible relationship to thunderstorm electricity. Phys. Rev., 78, 254-259.
2. 2) Reynolds, S. E., M. Brook and M. F. Gourley, 1957: Thunderstorm charge separation. Jour. Meteor., 14, 426-435.
3. 3) Magono, C., K. Orikasa and H. Okabe, 1957: The charge on precipitation elements and surface electric potential gradient. Jour. Fac. Sci., Hokkaido Univ., Ser. VII, 1, 7-20.
4. 4) Lueder, H., 1951: Emn neuer elektrischer Effekt bei der Eisbildung durch Vergraupelung in natiirlichen unterkiihlted Nebeln. Zeit. fur angewante Physik, 3, 247-253.
5. 5) Kuettner, J, and R. Lavoie, 1958: Studies of charge generation during riming in natural supercooled clouds. Recent Advances in Atmospheric Electricity, Pergamon Press, pp. 391-397.