constant voltage or amperage

Bryan L. Ford fordb at bcc.orst.edu
Wed Sep 17 12:39:40 EST 1997


js wrote:
> 
> Hi!
> If I can remember my phyiscs, then as the gel heats up during the run,
> Resistance increases. 

Unfortunately this is incorrect. Certainly, in solid conductors one will
generally see positive temperature coefficients, which John may be
remembering from his physics. But, for at least a couple of reasons, the
resistance of electrophoresis gels will *drop* with increasing
temperature (that is, there is a negative temperature coefficient).
Amongst the reasons are the the increased thermal mobility of ions in a
hot gel. Also the Arrhenius relation dictates that the degree of
ionization usually rises in aqueous salt solutions at higher
temperatures. David Spencer's suggestion that pore size may increase
with temperature may also contribute.

Personally, I prefer to regulate on constant "power" (watts). I would be
interested in people's objections to this both from performance and
safety perspectives. I always set the other controls (voltage and
current) so that they will default into regulation if the parameters
extend too far. But, I suspect that some newer digitally controlled
apparatus may not allow this sort of parallel default regulation.

-Bryan




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