These are based on the change in concentration occurring when a direct current is passed through a solution. The electrically charged particles migrate at different speeds according to the nature of the product.
These instruments usually incorporate a photometric device consisting of a photoelectric cell and a milliammeter graduated directly in units of optical density. They are used for analysing various solutions (proteins, amino-acids, etc.), for examining substances such as plasma, hormones, enzymes, viruses, etc., and for studying polymerisation phenomena.