Understanding Franklin's Wheel: A Van de Graaff Experiment

[amirali75]

Hey guys,
I'm new to this forum so I'm sorry if I posted in the wrong thread. So we did these experiments with van de graaff and I almost understood all of them except what seems to be "Franklin's wheel" experiment. I know for one thing that the air near the sharp points are ionized but I don't get why it actually turns and also why counterclockwise?
Any help is appreciated.

 

[AJ Bentley]

The sharp points create an intense local electric field which induces a charge on nearby atoms of air (oxygen, nitrogen etc.) by 'sucking' all the opposite charges away. The charged atoms (now called ions) are repelled by the field and shoot away like the exhaust of a jet engine. (This is called electric wind)
The reaction of that jet is what drives the wheel around.

Which way it turns depends on which way you have the points.