![Potentiometer](https://i.stack.imgur.com/3Xj5t.jpg)

So here's a picture of a potentiometer from my book. And my book says that the potentiometer is used for measuring internal resistance (of a cell), emf (of a cell) etc. But I cannot understand the concept behind this as how to find the emf of a cell? Also how to find the internal resistance of the cell? Could someone please explain this concept?