1. Central Research institute of Electric Power Industry, 2‐6‐1 Nagasaka, Yokosuka, Kanagawa 240‐01, Japan
2. Department of Science Education, Chiba University, 1‐33 Yayoi, Inage, Chiba‐shi, Chiba 263, Japan
3. Institute of Environmental Science and Technology, Yokohama National University, 156 Tokiwadai, Hodogaya‐ku, Yokohama, Kanagawa 240, Japan