This was originally asked by Sabre Tooth on Area 51 not too long ago. Perhaps it's relevant to put it on meta now that we're in beta, because although all the people here have clearly been on Area 51, not all may have seen the discussion. And it's good to make it official.
Anyway, should experimental physics questions be on topic here? The one answer, by martin (which got some upvotes) said that it should be on topic:
I'd definitly say that questions of the type "how can I build " would be on topic.
Remember, engineering practice may contain experiemnts too - measuring the performace curve of a pump would be one example.
Looking at experimental physics, I think questions of the type "I have this hypothesis, how can I test it" should be off topic. Here, I mean question were the focus is to translate the hypothesis into something testable in the first place.
For the record, I fully support these questions being on topic on Engineering (if they focus on the engineering aspect of a problem), and I have one planned. Cloud chamber experts be on the lookout!
Oh, and I would suggest that we generalize this to all sciences, not just Physics.