The use of copper and silver ions for purifying water has been practised for centuries. The ancient Greeks found that water kept in silver containers was mysteriously purified and that algae didn't grow in copper water pots.
Extensive research into the action of metals in the purification of water was not undertaken until the late 19th Century. This research resulted in the development of the 'Katadyne' water treatment process. This required that water be passed through various porous materials impregnated with silver. Some of the metal would become ionised and pass into the water as positive ions. As a result that water was purified.
It was discovered that over time 'Katadyne' porous filters became less effective and were difficult to clean. This resulted in the introduction of the 'Electro-Katadyne' process which involved an electric current being passed between silver coated sand beds and resulted in a better level of control over the ionising process.
The modern ioniser uses the same basic principle as the 'Electro-Katadyne' process except that the sand beds have been replaced with copper/silver electrodes and the ionisation control is accomplished with modern electronics.
The modern ioniser consists of two parts; the electrode assembly consisting of two (or multiples of two) bars of metal usually made of an alloy of copper and silver and the electronic control unit.