A conducting pinwheel is built with a arm which can rotate freely and bent sharp points on the ends. The device is placed on an insulating block and connected to one terminal pmshurst machine. The pinwheel will rotate away from the points for either sign of charge, but it rotates more rapidly for a positive polarity. The corona discharge from the point gives the air a space charge which repels the point because the polarity is the same for the point and the air.

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