When metals form ions, they give away one or more electrons. Some metals, such as lithium or sodium, lose their electrons very easily. Other metals, for example silver or gold do not give away electrons easily. The electrochemical series is a list of metals arranged in order of how easily the metal atoms lose electrons.
When metals give away their electrons, they do so with a certain force. This force is measured in volts. When two different metals are connected together in a cell, the metal with the higher force pushes its electrons on to the other metal. The image below shows what happens in a cell that is made from magnesium and copper in a beaker of ammonium chloride solution.
The two metals produce an electric current. Magnesium is higher up the electrochemical series than copper, so the magnesium can push away its electrons more strongly than copper. Electrons flow along the wires and through the voltmeter from the magnesium to the copper.
The voltmeter measures the force that pushes the electrons through the wires. If you replaced the magnesium in this cell with other metals you would obtain different voltage readings. By arranging the metals in order of their voltage readings, you can build up the electrochemical series.
The further apart the metals are in the electrochemical series, the higher the voltage. Electrons flow along the wire from the metal higher in the electrochemical series to the metal lower down.
To understand how electricity is made from pairs of metals the following points must be understood: