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These limitations are recognised... and there are others. We compensate for
the conductive/convective energy loss by extrapolating the vaguely linear
start of the power - T curve as a straight line, and then for several T
values we take the difference between the curve and the extrapolated line
below it as Pr, the radiated power. The glass envelope is kept immersed in
cold water to justify neglecting the energy radiated back to the filament
from the surroundings (I must check sometime to see just how much
difference this makes).
Strangely, the result from the log(Pr) vs log(T) graph is very consistent:
it invariably comes out as a straight line with slope somewhere between 3.5
and 4.5. A serious error analysis is probably too hard at IB level.
I regard the lab as a valid exercise: it could be improved by trying to
quantify better the difficulties that we bridge with simplifying
assumptions. I do have the feeling that there may be bigger errors than we
think, which compensate for each other to give a result which is a bit too
good.