Positron discovery I was reading about Carl David Anderson's discovery of the positron and I have a question.
I understand why it was classified as positive electron (due to its curvature direction and amount of the ionized vapor) but one thing I didn't understand was, why did it appear from below the chamber as opposed to other particles?
 A: As an old time experimental particle physicist , and having scanned thousands of (different bubble chamber) pictures , I think the answer lies in this quote from rob's  link:

A special feature of Anderson’s experiment was the introduction of a 6mm lead plate into the chamber, making it possible to decide if a particle was moving upwards or downwards.This was possible because the particle will get a decreased radius of curvature when it penetrate the lead plate and lose energy. Millikan had opposed to the introduction of the plate. He deemed it unnecessary because of his conviction that cosmic rays come from above, and nothing originating from the bottom of the chamber would be observed

The lead plate was left in the experiment and Millikan was wrong, because there are secondaries from high energy cosmic rays coming through the earth,mainly due to high energy cosmic neutrinos but also other backgrounds can exist.

Looking at the two curvatures one can see by eye that the curvature at the bottom is smaller than the one at the top, from energy conservation the track is entering from the bottom. If I were writing the original paper I would also show in parallel another frame where a track is entering from the bottom and is identified from the ionization  as proton or electron, for the contrast. But I suppose after it was established that the lead plate was crucial for determining direction, they did not see the need to stress the fact.
