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TC SECONDARY ELECT

 brad.alheim-at-the-spa-dot-com  writes: 

> Since the coil (form) has been "stressed" with a very high potential, it 
> will, being a "capacitor" store some of that electrical potential. Even 
> when discharged, it will regain some of its potential voltage due to this 
> "stress". I found this out (the hard way!) when I was designing computer 
> type monitors, and after discharging the CRT's, they would "charge them- 
> selves " back up again. This effect was exploited by the Japanese during 
> WW2, they had a portable transceiver that used a microphone with a wax 
> element that not only was a capacitive "electret" type, but provided 
> the necessary negative grid bias from the stressed electrical potential. 
> (I later found one of these little transceivers, but stupidly sold it!) 
> I do not know the exact scientific "physics" explaination as to why the 
> electrical stress on a dielectric will do this, perhaps someone out 
> there can provide a more "scientific" dissertation!   

You've described it pretty well, actually. The term I've seen used in 
fields textbooks for the "stress" you mention is polarization. It's 
reasonably analagous to magnetization, where a nonmagnetized peice 
of material can be magnetized. 

Since dielectric materials contain charge, putting an electric field 
through them (happens by putting a voltage across them) will cause  
its charge to be redistributed. Since the dielectric tends to be an 
insulator, the charge in individual molecules remains with its molecule,  
though the molecule may be slightly deformed. This means that the actual 
movement of charge in a single molecule is very tiny. In the case of polar  
molecules, those which actually have a positive and negative "side",  
the molecule may rotate slightly, in a direction that helps align  
itself with the field. The slight movement of charge in a single  
molecule has little effect, but the combined effect of the movement  
due to all the molecules in the field can be significant, and looks 
externally as though there's charge on either side of the dielectric. 
When the field is taken away, the molecules "relax", and the effective 
charge vanishes.  

Now, for some materials, I think it's for some polar materials only, but  
I could be wrong, the molecules can be allowed to move more freely  
when the material is heated, allowing them to be better aligned under an  
electric field. If the field is retained and the material allowed to  
cool, the molecules are "frozen" in their aligned position, and you have 
an electret, the electric field equivalent of a permanent magnet. A  
permanent electric field will exist in and around this material, which 
means that a permanent voltage will exist across it. I guess this could 
be used for the grid bias in the transceiver you mentioned. It sounds 
like a clever application for the material. 

Is this the kind of explanation you were looking for? 

By the way, on the subject of getting shocked by TC secondaries, it 
may be useful to bring your questions to an ESD (Electrostatic  
Discharge) expert, if you have one at your place of employment. I 
worked some time back with laser diodes that were very ESD sensitive, 
and had some interesting chats with an ESD guy. He showed me how  
he could wave his little field strength meter around a peice of  
plastic and show nothing, then get himself charged up and touch the  
plastic in a certain spot, thus causing only that area in the plastic  
to have a significant electric field. He mentioned that if he checked  
the spot tomorrow, he might still get a reading. 

This all sounds a lot like the discussion that's been going on here 
so far, and while my anecdote may not add anything new, it may point 
to a type of person who might have insights on the phenomenon. 

Wes B.