A type of Battery

MIT的教授上課示範怎麼用水產生電力看影片...水能生電?? 誰能告訴我為什麼?
http://www.rmcybernetics.com/projects/experiments/experiments_kelvins_thunderstorm_electrostatics.htm
http://www.newphys.se/fnysik/3_1/kelvin/index.html