metals came into contact with certain chemical solutions, voltages appeared betweenthe pieces of metal. These were the first electrochemical cells.A piece of lead and a piece of lead dioxide immersed in an acid solution (Fig. 7-1)will show a persistent voltage. This can be detected by connecting a galvanometer be-tween the pieces of metal. A resistor of about 1,000 ohms should always be used in se-ries with the galvanometer in experiments of this kind; connecting the galvanometerdirectly will cause too much current to flow, possibly damaging the galvanometer andcausing the acid to “boil.”Primary and secondary cells1197-1Construction of a lead-acid electrochemical cell.The chemicals and the metal have an inherent ability to produce a constant ex-change of charge carriers. If the galvanometer and resistor are left hooked up betweenthe two pieces of metal for a long time, the current will gradually decrease, and the elec-trodes will become coated. The acid will change, also. The chemical energy, a form ofpotential energy in the acid, will run out. All of the potential energy in the acid will havebeen turned into kinetic electrical energy as current in the wire and galvanometer. Inturn, this current will have heated the resistor (another form of kinetic energy), and es-caped into the air and into space.Primary and secondary cellsSome electrical cells, once their potential (chemical) energy has all been changed toelectricity and used up, must be thrown away. They are no good anymore. These arecalled primary cells.