I love that Tom doesn't have to prove any of his 'positive' claims about errors. This is precisely why the negative/positive distinction is not a logical tool, but is merely a cognitive dissonance in the minds of people who are unwilling under any circumstances to alter the beliefs they consider precious.
We can always reformulate the discussion in the opposite direction. Tom, you are making a positive claim: "
X can/does cause an error in your measurement/experiment."
You have to prove that claim. By your own logic, it's obviously impossible for anyone to prove that the experiment does
NOT have a source of error. That's a negative claim, and it can't be proven, remember? We have to assume that it does
not suffer from error until an error has been proven.
Prove away...
e: Oh, here's a technical document and specs for the Karn EWB 2.4 scale that took me all of 10 seconds to find on Google. It's a good thing I don't share your view that it's always someone else's responsibility to teach me new things and not learn anything for myself.
http://www.inscale-scales.co.uk/pdf/eg-m.pdf