I have trimmed the tubes to equalize their inner volume to about 12 ml. I am now repeating yesterday's experiment, but using south poles. I took all mesures to avoid the previously mentioned pitfalls.
Here are the results.
South-pole treated samples slightly outperform untreated samples; the difference is not very significant. But there is more.
First, although I trimmed the larger, green silicone tubes so that they have the same volume as the others, they still consistently give higher results -- see the upper two curves.
Second, there is significantly more variability in the control group than in the treatment group. I think this might be due to light. I simply put a drape over the flasks in an attempt to uniformize the amount of light they receive. However, the higher control flask that is not using a large tube sits in the shadow of a distant compact fluorescent lamp, while the other flasks do not.
Tomorrow, I will build an opaque box to protect the flasks.
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