Turning the crank on the Wimshurst machine leads to large arcing sparks between the ball shaped electrodes. For every centimeter traveled in air the voltage required is 31,000 Volts.
Air friction due to the spinning glass disks leads to ionization of the surrounding air. These ions stick haphazardly to the disks, being attracted or repelled by any charges already present on the disks. Eventually the machine becomes charged as shown in the schematic on the left (with cylinders instead of disks illustrated for ease of view). The charging results because any small charge variations existing on the disks respond to a positive feedback mechanism. Note the conducting bar that connects the lower negative aluminum strip on the inner cylinder with the positive upper strip. The particular charge separation shown (positive up and negative down) occurs on contact between the metal strips and bar due to the attraction of opposite charges. The charges producing the attraction for the inner cylinder, however, are the charges on the outer cylinder. Due to this charge attraction and conduction, the inner cylinder becomes positively charged on all strips on the upper right half, from bar end to bar end, and negatively charged on the lower left half, consistent with the clockwise rotation. Of course, the charge on the inner cylinder attracts charge on the outer cylinder in a similar fashion. The counter clockwise rotation and bar orientation produce the charge separation shown for the outer cylinder. Each disk induces the other to charge properly, a situation called positive feedback. Consequently, the disks build up more and more charge as they spin.