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Long exposure photographs of Wimshurst generators taken in the dark reveal how they work and provide valuable insights into perfecting electrostatic generators and motors. The Wimshurst machine was first described in 1883. Despite its age and popularity, descriptions of how they work are often incomplete. As such, various components including sector shape and spacing, charge collector geometry, neutralizer bars, and discharge ball size, have evolved over time somewhat by trial and error. A better understanding of how these generators work can lead to better designs.  Long-exposure photographs taken in the dark reveal plasma conduction paths and other processes that shed light on how to improve generator performance. The following publications give a good and relatively-modern summary of experiments and observations. 1.     Operation of the Wimshurst Machine   Last updated: 2018 Dr. Antônio Carlos M. de Queiroz   COPPE/UFRJ - Programa de Engenharia Elétrica,CP 68504,21941-972 Rio de Janeiro, RJ, Brazil 2.     Homemade Lightning, Creative Experiments in Electricity, 1996 by R.A.Ford 3.     A double Wommelsdorf condenser machine

Publication 1 states: “The Wimshurst machine is an "influence machine", an electrostatic generator that uses the charges already present on it to generate more charges by electrostatic induction.” Long exposure photographs of a working Wimshurst generator taken in the dark reveal there is much more going on than just electrostatic induction. The generator produces plasma due to corona discharge. This creates conduction paths that charge the ends of the neutralizer bars. These conduction paths also lose energy and can limit the voltage output of the generator. The following long exposure picture of sparks and corona discharge reveal the location of these conduction paths.

Figure 1. This 30 second, ISO 3200 picture taken with an Android smart phone shows the location of the corona discharge conducting paths in a Wimshurst generator. The green lines show the position of the charge collectors on the left and right, the position of the neutralizer bars and the inside edge of the sector plates.	Figure 2. The position of the charge collectors,  neutralizer bars and sector plates are shown in this picture taken with the room lights on.

Figure 1 and 2 contain a wealth of information about how a Wimshurst generator works. Typical descriptions describe how the Wimshurst self-charges through induction across the front and rear sector plates. See Figure 5. Wikipedia, for example, describes this self-charging process, but states that it is not enough to explain the high voltages that are actually produced. A second process is needed that involves the potential difference across the left and right charge collectors. This paper describes that process in more detail. The large potential difference across the charge collectors charges the ends of the neutralizer bars. The corona discharge pattern in Figure 1 reveals how conductive plasma forms between the inner radius of the sector plates and how these conductive paths charge the ends of the neutralizer bars.

Figure 3. This long exposure picture reveals which side the corona discharge paths occur on. Plasma on the rear disk has a slightly smaller radius. Also the front neutralizer bar blocks light from the plasma on the front disk while the rear bar does not.	Figure 4. Plasma on the rear disk has a slightly smaller radius. Also the front neutralizer bar blocks light from the plasma on the front disk while the rear bar does not.plates and when they lose contact with the sector plates leaving a charge on the plates.

Figure 5. Typical descriptions describe how the Wimshurst uses charging by induction through the front and rear sector plates. The front sector plates near the red curve shown here must be positively charged to charge the right positive collector. They gain their charge at the front neutralizer bar on the left and lose their charge at the right collector. These positively charged plates induce a negative charge on the adjacent rear sector plates when the brush on the right rear neutralizer bar touches a rear plate. This is shown with the dashed blue curved arrow. The arrow shows where they become charged and where they lose their charge at the negative charge collector on the left. The capacitance between these plates is not enough to explain how the Wimshurst produces such high voltages so a second process must be occurring.

By understanding how the Wimshurst works, we can optimize the various Wimshurst components for various design goals. Do we want the highest possible voltage, or highest current or the best efficiency? These components include: sector geometry, charge collectors, neutralizer bars, disk size, discharge ball size and inherent asymmetry. Two of these, the charge collectors and inherent asymmetry are described here. Others will be described in future publications.

The Remarkable Design of the Charge Collectors

Figures 6: An edge-on view of a U shaped collector is shown in yellow. Green electric field lines and white potential lines are shown. Sector plates move from left to right. Positively charged plates are red. Neutral plates are white. “Dipole” plates with separated negative and positive charges are blue and red. Plasma conduction paths are shown in white. Field and potential lines were produced with the Electrostatics Simulator written in Java by Paul Falstad. http://www.falstad.com/

Mysterious Inherent Asymmetry

The asymmetry is due to the difference in mass between electrons and air molecules. The Wiki article on Corona Discharge states:

“The physics of positive and negative coronas are strikingly different. This asymmetry is a result of the great difference in mass between electrons and positively charged ions, with only the electron having the ability to undergo a significant degree of ionizing inelastic collision at common temperatures and pressures….  The difference, then, between positive and negative coronas, in the matter of the generation of secondary electron avalanches, is that in a positive corona they are generated by the gas surrounding the plasma region, the new secondary electrons travelling inward, whereas in a negative corona they are generated by the curved electrode itself, the new secondary electrons travelling outward.”

Our own observations revealed that we can balance this asymmetry by adding a wire to the negative charge collector and thus increasing the voltage output. We used a parallel plate scale voltage meter as described by Physics Guys at PhysicsGuy.Org. Adding a wire to the positive side decreases the voltage output. Asymmetry can be measured by using a parallel plate scale voltage detector where both plates are connected to the positive charge collector. Then this is done with negative collector. Experiments show the positive terminal has three times the voltage magnitude compared to the negative terminal.

Many publications report a similar balancing effect where adding a smaller ball to the positive terminal produces longer sparks which indicates higher voltages are generated. They hypothesize that the production of longer sparks is due to the concentrated field around the smaller discharge ball, but there may be more to it. These observations indicate that changing the self-capacitance of either terminal changes how well the Wimshurst generates voltage.

Here are Dr. Antônio Carlos M. de Queiroz’s observations:

“Significantly longer sparks can be obtained by the addition of a smaller ball attached to the positive terminal. This increases the electric field around the positive terminal, forcing the ionization of the air to start at the positive side of the gap, instead of at the negative side, as happens with a symmetrical gap. The positive ionization forms a nice plume-like structure (that can be seen in the dark) pointing away from the positive terminal, that easily gets connected to the diffuse corona that the negative terminal emits. When the connection is established, a spark occurs, with the electrons flowing from the negative terminal to the positive. Short sparks are observed as bright straight lines. Longer sparks are more irregular, and may present branches, almost always in the direction of the negative terminal. In short sparks, the positive end is brighter, specially in sparks obtained without added Leyden jars, due to more concentrated current. In long sparks, the positive end is always a straight line perpendicular to the surface of the terminal, with the negative side irregular and a little brighter (why is a mystery).”

Verifying the Charging Mechanism

Figure 8. Copper foil plates were placed on a plastic board so that the edges were one cm apart, about the same distance as the sectors on the Wimshurst disc. Even though the edges did not touch at all, experiments proved that a current across the air gap develops when a high voltage is applied to the plates on the left. When plastic tape is placed over the gaps so that the edges are covered in plastic, there is no current

A Wimshurst generator could be placed in a vacuum chamber to eliminate or reduce conduction paths. An alternative approach would be to cover the sector plate edges on the generator with an insulating plastic as was done in Figure 8. Publication 3 describes a Wommelsdorf Condenser Machine where this was done. The result is the Wommelsdorf produced higher voltage and was more efficient. Much higher voltages are needed to create conduction paths through the dielectric plastic used in this generator. A standard Wimshurst generator is 15% efficient and a Wommelsdorf Condenser generator is 32.5% efficient.