| MadSci Network: Physics |
First off, this question has been a good learning experience for both of us - I knew what luminglass (or plasma globes) were, but I had never figured out how they worked. Don't let this scare you though, because I have done some web research, poured over my old textbooks, and talked with a good friend of mine to find you an answer to your question!
Before you read this, you may want to go and check out the websites I listed at the end of my answer. One of them is a U.S. patent (the one called "4754199.tif Page 1"). It has some very good diagrams and is what I chiefly used to formulate my answer.
Plasma globes consist of a high voltage / high frequency power source, a dielectric globe (e.g. glass) coated with a conductive film, and ionizable gas (e.g. neon) to fill the globe with. The power source is connected between the electrode (that ball on a stick, in the middle of the globe) and the conductive layer on the globe's surface.
The high voltage electrode will create an electric field that extends radially from the electrode outwards - even outside of the globe. (like rays of light extending from the sun or a light bulb) See Figure 1 below:
| / --
| / ----
\ | / --- ----
\ | / ---- ------
\ | / --- ------ electric field lines
\ | / ---- ------ -----
\......./-- ------ --------
.. .. ---- --------
. . --------
. electrode . ----
. . --------------------------------
.. .. ------
/ ....... -- ----------
/ | ---- ----------
/ | ---- ------
/ | ----
/ | ----
/ | ----
--
Figure 1: Electric field lines extending from electrode
The high voltage will have so much energy that it will ionize the gas in
the globe, creating a plasma (a cloud of charged particles). In the region
of the ionized gas (or plasma) there will now be a much lower resistance
between the electrode and globe surface. Thus, an electric arc is created
between the electrode and the globe to complete the circuit. The arc
extends outwards, following the electric field lines. This apparatus can
be represented electrically by the following diagram in Figure 2:
+-----------+
| | where: V = AC voltage
| +-+-+ R1 = resistance between electrode
V |R1 | and ground, or globe (across the gas)
| | |
| +-+-+
| |
+-----+-----+
--+-- Figure 2: Equivalent Circuit for Plasma Globe
---
- (Ground)
This explains how all the little arcs of electricity are created within the
globe, but it still does not tell us how much larger arcs are formed when
you put your finger near (or touch) the outer globe surface. When you
touch the globe you are in fact adding your self into the whole plasma
globe electric circuit. This is not dangerous though. Due to the high
frequency of the power source, not enough amplitude (or power) is built up
in the signal to ever shock you. I will explain how this works......
Remember the electric field that extends through the glass globe? Well, when you touch the outer globe surface, your body, the glass, the inner conductive layer, and the electric field all come together to essentially create a capacitor! (neat, eh?) The electric field creates a voltage (or potential) between the globe's inner conductive layer and your body (the other "conductive layer"). Your body also provides some resistance between it and the ground. Put all this together and you get for following equivalent electric circuit in Figure 3:
||C
+-----------+-----||------+
| | || | where: V and R1 = same as above
| +-+-+ +-+-+ R2 = resistance through your
V |R1 | |R2 | body to ground
| | | | | C = capacitor created between
| +-+-+ +-+-+ the globe conductor and
| | | you
+-----+-----+-------------+ (R1 and R2 are very high
| resistances - around 10,000
--+-- ohms or more)
---
- (Ground) Figure 3: Equivalent Circuit for you touching
the plasma globe
If you now do some analysis of the electric circuit (which I will not show
here - contact me if you want more) you will see that at very high
frequencies, the capacitor has very little resistance (or properly termed
capacitive reactance). When this capacitive reactance is properly summed
with the resistances of the globe conductive layer and your body (R1 and
R2), you will notice that there is a much lower resistance at this point
where your finger is than there was before it was there. This lower
resistance makes a very attractive site for an arc from the electrode -
because electricity always likes to take the easiest way to ground. Thus,
a much larger arc is formed where your finger is!
Well that just about does it. That is a quick explanation of how a plasma globe works. Once again, if you have not already, check out the websites I listed below. They will help you to investigate this topic further. I also listed the texts that I referenced.
I hope this answer sufficiently explained the workings of a plasma globe to you. If you have any further questions or if something I wrote just does not make sense, feel free to email me.
Good luck!
Kurt Frost, kfrost@sympatico.ca
Try the links in the MadSci Library for more information on Physics.