The simplest design consists of two electrodes made from metals of different reactivity immersed in an electrolyte and connected to an external voltmeter by wire, creating a complete circuit
A common example is zinc and copper
Zinc is the more reactive metal and forms ions more easily, readily releasing electrons
The electrons give the more reactive electrode a negative charge and sets up a charge difference between the electrodes
The electrons then flow around the circuit to the copper electrode which is now the more positive electrode
The difference in the ability of the electrodes to release electrons causes a voltage to be produced
The greater the difference in the metals reactivity then the greater the voltage produced
The electrolyte used also affects the voltage as different ions react with the electrodes in different ways