WARNING: This summary deals with open flames, dangerous voltages,
cold and hot pieces of glass (which may cut and burn you).
As usual I'm not responsible
for any injuries concerning yourself (or other people) or damages to your equipment
and property (or the property of other people) caused directly or indirectly
by anthing on this website. YOU HAVE BEEN WARNED.
Electrically heated glass
I heard that glass, which is an isolator at normal temperatures will get
conductive if heated to red heat. Of course, I had to try this.
Prerequisites
At first, I needed some glass. Additionally I needed some something to heat
it till it was conductive enough to hold the temperature by passing current
through it. I used a propane torch which is not the best way
(would be easier if it was hotter) but sufficient. The last thing is current
(and voltage of course, as we need power).
I expected to need more power than my bench supplies can deliver so I
used a phase controlled modulator (aka a dimmer) which got plugged into outlet.
Additionally it is always a good idea to use AC with wich setups because with
DC the ions may get seperated. (Well, I don't know if that is true for glass
but with liquids it is.)
First try
I fired up the propane torch and heated a drop of glass which I placed between
two steel electrodes (paper clip wire in my case), because the laborytory clips
should stay cool. That's why I used long, thin electrodes which allow a
temperature gradient to build up along them.
I switched on the dimmer and cranked it up as little as I could and - Peng!.
The dimmer's fuse blew (and the triac too. Doh!). It seems like the glass
heated up a bit, making it more conductive, which heated it up, which made
it more conductive,...
Within a fraction of a second, just before the fuse blew, a small white
(yes, white not red!) glowing spark jumped out of the glass drop.
Of course I expected the glass to have a negative temperature coefficient,
that's why it gets conductive, but not that extreme.
Second try
Obviously I needed some sort of current limiting. I tried a 100W light bulb
in series but I felt that more power would be interesting so I switched to
a 750W hot plate (which was kept cool by a pot of water onto it).
Note that only a fraction of this power goes into the glass. The voltage
drop of the glass is not very high so it would have been more efficient to
use a lower voltage source but I had nothing matching.
I wound the electrodes spiral
like to keep the glass from flowing away and dripping down. (It still flows
somewhat but I simply put a new piece of glass onto the top electrode
from time to time which melts and compensates the loss.)
If run very hot the glass drop glows so
bright that looking into it is very uncomfortable.
Notes:
- You really need some way to control the amount of current. The reason is
that the glass gets more conductive if its hotter. This leads to the problem
that only a small channel in the glass will carry all the current. This
gets worse if you don't increase the current slowly. (I havent tried what
happens if I apply full power immediately. Maybe some gas bubbles will form
in the glass but I'm not a fan of hot glass which sputters around so I won't
try that.)
- Be very careful if you try it yourself. You are playing with
about 1300K hot glass and harmful voltages and open flames.
Further tests
I've hooked up a voltmeter and current meter. The voltage is always in the
range of 15V to 25V (with my electrode and gap size). At maximum power it runs
at about 20Vrms and 3Arms and thus consuming about 60W (if there is no
phase angle between current and voltage. I didn't measure that but it should
act mostly resistive as it has to dissipate heat.).
These 60W
are enough to keep it glowing hot and bright because there is only a small
amount of heat transfer.
The glass turns first greenish and then black after a while because of the
impurities from the wires etc. which accumulate in the glass.
Pictures
First try
Later tries
Home
Copyright (C) 2008 by Wiesner Thomas
Last change: April 11th 2008
