Re: IONS

["Tim Folkerts" <tfolkert@bigcat.fhsu.edu>]

Just to be practical, I went to the CRC handbook and found some numbers.

For silicon oxide
        20 C        rho =  1 x 10^13 Ohm*m
      600 C        rho =  7 x 10^4   Ohm*m
    1300 C        rho =  4 x 10^-3  Ohm*m

As has been mentioned, real glass will have impurities which lower the
resistivity.  Another reference shows that at 700 C, the resistivities
of pure alumina (99.9%) and 85% pure alumina differ by three orders of
magnitude.  Thus it is quite believable that impure silica would show
similar differences.

It is easy to pick a reasonable size rod (say 10 cm long with area = 1
cm^2) and to pick a reasonable resistivity (say 1 Ohm*m) and get powers
of a few tens of Watts from 120V AC.  I may have to try it some time.

Tim Folkerts




Ludwik Kowalski wrote:

> The rate of cooling of the glass rod at 500 C is probably
> large, perhaps 30 W. This would mean that to sustain the
> constant temperature the electric current must be large.
> More specifically, i=30/120=0.25 A and R=480 ohms. Very
> small in comparison with perhaps 10^9 ohms, or more, at
> room temperature. Perhaps the "glass" chemists use is not
> pure. But how can it be; they are in the best position to
> know.
>                          Ludwik Kowalski