This question is an exact duplicate of:
I had asked this problem earlier but I was too vague in how I asked it so I'm making a new post to be more specific.
Problem: Say we have a series circuit with an ammeter, a cell with internal resistance and known emf, and a 10 ohm resistor. It is noticed that the resistor gets warmer. How would this affect the calculated value of the internal resistance?
Here is my thinking: Since the resistor increases in temperature, it increases in resistance. Let's look at the equation ℰ = IR + Ir, where "ℰ" is emf, "I" is current, "R" is external resistance, and "r" is the internal resistance of the cell. if R increases and current is constant, in order to maintain constant emf, r must decrease. Where's the flaw in my thinking?