Question

sources of error while verifying ohm s law

Answer 1

Ohm's Law only applies to a very limited range of 'ohmic' or 'linear' conductors, despite what Ohm thought when he wrote about it. 
So, it is hardly a 'law' at all -it certainly isn't universal. Quite frankly, there's very little point in continuing to teach Ohm's Law, so it's surprising how much effort is spent on doing so