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Registered Member #350
Joined: Mon Mar 27 2006, 05:14PM
Location:
Posts: 106
Hi, This seems like a strange question. I read that in the 19th century people made static electricity generators from glass. But the strange thing is that besides of tv screens I've never ever seen a piece of glass getting a static charge. So I tried to find out why. I took a glass bottle made from plain clear glass. I cleaned it and used a hair dryer on it to make sure it's perfectly dry since glass tends to attract moisture from the air. Then I tried to charge it. First by rubbing and then with a HV generator. It didn't work. When I touched the wire to the glass I could hear a hissing noise like when I touch the wire to wood but the glass remained uncharged. With a plastic bottle however it works fine. There I also get a hissing noise but it only lasts for a moment until the plastic is charged up. So this glass was obviously conductive. Is that the case with most kinds of glass?
Registered Member #902
Joined: Sun Jul 15 2007, 08:17PM
Location: Pacific Northwest USA
Posts: 1042
I don't know the percentage of glass that is, but I had this happen when making a wine bottle capacitor... haven't tried too many types of glass though...
Registered Member #834
Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644
Pure silica is an excellent insulator, but has a too high melting point. Sodium carbonate is then added to silica to lower the melting point and make the most common glass. A problem is that this makes the glass surface slightly conductive. Pyrex glass has a different composition and is a better insulator. It is possible to use regular glass in an electrostatic machine, but some surface treatment is necessary. The application of some wax to the surface is enough to make a big difference.
Registered Member #350
Joined: Mon Mar 27 2006, 05:14PM
Location:
Posts: 106
I read some old texts that were written in the 19th century on books.google.com. They clearly used plain untreated glass in many experiments and as long as the air humidity was low it was a very good insulator and charged when it was rubbed. The type of glass used was never mentioned so I assume it didn't matter. I did some more tests and the only non conductive glass I could find in my home was on my old TV. That makes sense. A CTR tube must of course be made from highly insulating material.
Registered Member #193
Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022
All common glasses are, and always were, insulators. The ions are not mobile in glass unless you heat it up . However most glass is, to some extent, hygroscopic and the film of moisture that forms on the surface is a good enough conductor to leak charges away.
Registered Member #350
Joined: Mon Mar 27 2006, 05:14PM
Location:
Posts: 106
Thanks for your answers but I already knew that glass is hygroscopic thats why I used a hairdryer on it. Anyway, I tried again to make a glass bottle insulating by cleaning it with acetone and blowing it dry with a hair dryer. But it stayed conductive. So I have to conclude that bottles are made of some kind of glass that is either conductive or has a conductive surface. The second option seems more likely. It can't be due to moisture though so it must be something else. Something that even acetone can't clean off. Some kind of surface treatment maybe? Or a side effect from the production method, e.g. tin being absorbed into the glass from the "float glass process". If bottles are made out of old window glass they will contain some tin. Also the surface of glass bottles is often chemically treated so it won't react with liquids inside. But I think that process would rather make it more insulating and not less.
Anyway, this means I can not make a leyden jar out of a glass bottle. So I will just have to stick to plastic.
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Is all glass slightly conductive nowadays?
