Polarography is a method of chemical analysis, particularly for metals or organic groups, that can measure tiny concentrations of ions in solution. Two electrodes are inserted in the solution: one, the reference electrode, is kept at a constant voltage, the other has a variable voltage and consists of a glass capillary tube containing a head of mercury, with tiny drops of mercury on its end becoming the face of the electrode. As these drops form, a current grows to a peak between the two electrodes and then suddenly falls as the drop leaves the capillary tip. By gradually increasing the voltage, a polarogram – a graph of the current versus the voltage, which looks like a series of steps – is produced. From these, the reduction potentials of the ions in the solution may be deduced and, as these are known for each metal ion, the ions can be identified.