The Wimshurst machine is one of the oldest and best known electrostatic machines, consisting of its iconic two counter rotating disks and two Leyden jars. Most often you see someone hand cranking it, producing sparks, though we’ve seen it used for much more, including for powering a smoke precipitator for cleaning up smoke and even for powering a laser.
It works through an interesting sequence of events. Most explanations attempt to cram it all into one picture, requiring some major mental gymnastics to visualize. This often means people give up, resigned to assume these work through some mythical mechanics that defy a mortal’s ability to understand.
So instead, let’s do a step-by-step explanation.
The Beginning: Charging The Sectors
Each disk is covered in metal sectors on their outward facing sides. The sequence of events begins at any sector that has an unequal amount of positive or negative charge. As long as the sectors are clean and dry then there’s usually at least one that’s charged. Let’s say for example that one has a net negative charge and is on the front disk.
That net negative charge influences the nearest sector on the rear disk, repelling negative charge to the far side of it leaving the near side with a positive charge. That’s called electrostatic induction, and it’s for that reason that the Wimshurst machine is called an influence machine since the charge on one sector influences the charge distribution on another sector.
Next, let’s switch to the rear disk and look at what happens to that sector that’s been influenced.
The next thing that happens is the real genius. Each disk has a neutralizer bar facing it. Each end of a neutralizer bar has a brush that touches the sectors as they pass. And there are an even number of sectors. That all means that when a brush is touching a sector, the neutralizer bar is now electrically connecting that sector with another sector at the other end of the neutralizer bar. It shorts them out.
Let’s say a neutralizer brush is touching the sector that’s been influenced, the one shown above that has had its charge redistributed such that it’s positive on the side facing inward and negative on the side touching the brush. Even though the sector is neutral overall, the neutralizer bar sees only the side that’s negatively charged. It now sees an imbalance between the two sectors that its two brushes are touching. That causes a current to flow in order to restore that balance. Some of the negative charge will flow from our influenced sector to the other sector. From the neutralizer bar’s persepective, it has now neutralized the charge on the two sectors.
When the disk rotates the sectors away from the neutralizer bar, the first sector is left positively charged having just had some negative charge taken from it. And having received that negative charge, the other sector is left negatively charged.
These charged sectors are rotated more to where they face sectors on the front side of the disk just when those sectors are touched by the brushes of the neutralizer bar on that side. And so the newly charged sectors influence charge in more sectors, and so on.
A helpful realization is that this influencing and neutralizing event causes one sector to make the sector facing it on the other disk become charged with an opposite charge. Our negatively charged sector created a positively charged sector. That positively charged sector, once the disk was rotated, went on to create a negatively charged sector.
Charges Whirling To Collectors
The front and rear disks (which are rotating in opposite directions) result in the charges as shown above.
It may take a moment to convince yourself (since you’re seeing the front and rear views side-by-side), but all negatively charged sectors are headed to the left collector and all positively charged sectors are headed to the right collector. You’ll also notice that the sectors that have just passed the collectors have had their charges, well, collected. They’re neutral overall again until they get to the neutralizer brushes, where the influencing and neutralizing we covered above recharges them.
The collectors don’t touch the sectors. Instead they have sharp points that face the sectors and have an air gap between them. This is a familiar technique which we’ve seen before in the functioning of Van de Graaff generators. Each collector has sharp points facing sectors on both disks which facilitate the transfer.
Using the left collector as an example, the negative charge on the sectors repels electrons from the points, leaving behind a positive charge. Since they are sharp points, that positive charge is crammed together resulting in a strong electric field in the gap near the points. That strong electric field tears air molecules apart and begins the process of making the air conductive, forming a bluish corona near the points. It’s that conductive air that causes the negative charge of the sectors to cross the gap to the collector. That leaves the sectors neutral again.
A similar thing happens at the right collector, just with opposite charges. Since those sectors are positive, the sectors will receive electrons from the collector, making those sectors neutral again.
But where does all that charge used for neutralizing the sectors go to and come from? That’s where the rest of the circuit plays a part.
The Leyden Jars And Spark Gap
The rest of the circuit consists of a spark gap and two Leyden jars. The two Leyden jars are just two cylindrical capacitors connected in series. The spark gap can also be thought of as a capacitor, albeit one with a dielectric that breaks down easily and that has a low capacitance compared to the Leyden jars. The spark gap is in parallel with the Leyden jars, and both are in parallel with the collectors.
That means the collectors are connected to each other through the disks but also through the Leyden jar/spark gap capacitors.
Charge that’s collected from the sectors charges up the Leyden jars and the spark gap. The Leyden jars are designed to withstand a higher voltage than the spark gap so it’s the spark gap that breaks down first. When it does it produces a short circuit. All the accumulated charge in the Leyden jars quickly dumps through the spark gap as a spark, neutralizing the Leyden jars until the charging process starts again.
In summary, through induction, the neutralizer bars are tricked into charging the sectors. The collectors collect that charge and store it in Leyden jars and the spark gap. When the charge has resulted in a sufficient potential across the spark gap a spark occurs, shorting out the Leyden jars until enough charge can be collected again for another spark.