EMF of cell can be defined as the actual potential difference of the cell. The voltage that we are getting from the battery is reduced by the internal resistance of the battery. Circuit diagram shows is internal physical structure of the cell. It contains a resistor and the cells voltage. Resistor that is connected here is not seen as this is not connected in the cell or it is outside the cell. It is the resistor that is of the cell. And emf of the cell is constant means that the emf of the particular cell does not change. Aluminium foil is used in this project. Lots of aluminium foil is wasted in the garbage after using it in the lunch box. This will give you aluminium free. You need not purchase it from shop asks your friend to give his used aluminium foil, It is eco-friendly. This project can be made anywhere. As it does not require any precious metal.