The device which produces electrical energy from chemical reaction is called electrochemical cell. The best example of an electrochemical cell is the Daniell cell in which copper and zinc electrodes are immersed in a solution of copper sulfate zinc sulfate respectively. Here zinc acts as anode and copper acts as cathode. Potential difference between the two electrodes is called cell potential or emf of the cell. It is measured in volts.
This video explains how to study the variation of EMF in the zinc – copper electrochemical cell with change in concentration of electrolytes at room temperature.