I'm doing an experiment to measure the internal resistance of a zinc carbon cell (battery) and have slightly confused myself! Do I use the ε= IR + Ir formula? And if so how in the world do I use it! I know that I can work out R by using V and I values but how do I find the EMF of the battery? And what value do I use for I cos I have hundreds cos current isn't a constant! I've royally confused myself!