why does hauksbees electrostatic machine produce light?

why does hauksbees electrostatic machine produce light?


1 Answers

Mukesh Sharma
askIITians Faculty 43 Points
10 years ago
If you model the globe as a simple insulating circular glass shell with a constant spatial charge density embedded in the glass in the immediate vicinity of the finger and solve Maxwell's equations numerically for the potential, you observe something like this:

enter image description here

The rationale for placing a nontrivial static charge density in the vicinity of the finger in contact with the spinning globe is simply because glass, the so-called "vitreous" static source, dislodges surface charges during mechanical abrasion, and the charges have nowhere to go, since glass is mostly nonconducting.

As you can see, on the inside and outside of the glass in the immediate vicinity of the finger, there is a potential gradient. As such, dielectric breakdown becomes a possibility. By Paschen's Law, dielectric breakdown is orders of magnitude more likely in the partially-evacuated interior of the globe; as such, surface charge in the vicinity of the finger on the interior of the globe are accelerated through the vacuum and redistribute themselves on the lower-potential walls of the globe farther away from the finger. The electron collisions with nitrogen and other rarefied molecules in the vicinity of this transient current generate the blue glow near the finger.

Since the globe is spinning and the finger continually moves across the globe, there is no need to worry about a net nonphysical change in the interior surface charge density during each traversal of the loop, since they are constantly being "spatially recycled" during the spinning cycle.

For this reason, I am willing to bet that a similar experiment conducted with a finger rubbing the same spot on an evacuated glass bulb will NOT produce a continual glow discharge in the vicinity of the finger, despite the mechanical similarity of the processes.

Finally, assuming my mechanism is correct, there should be no visible glow discharge if the glass bulb is near-perfectly evacuated. Vacuum techniques in Hauksbee's time were, of course, far from this degree of rarefaction.

Thanks and regards

askIITians faculty

mukesh sharma

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