POLARIZATION. If a simple voltaic cell is put in circuit with a galvanometer it may lie ob served that the current gradually diminishes. tf now the adhering film of hydrogen be brushed off the copper plate with a glass rod, the current will increase to nearly its original value. This decrease of the current is due to several causes which are included under the term polarization. The hydrogen at the positive electrode is the chief agent in decreasing the electromotive force of the cell, by means of the negative or counter-electro motive force which it produces.
To prevent polarization moans are adopted to replace the hydrogen ions with others, such as copper or merpury, which do not produce polar ization; or else the positive elect rode is sur rounded with a chemical which furnishes oxygen or chlorine to unite with the hydrogen before it reaches the electrode.
TuE DANIELL CELI„ invented by Professor Daniell, of London, in 1836, was the first suc cessful attempt to prevent polarization by chem ical methods. it con sists of a glass jar .ontaining a saturated solution of copper sul phate (CuS(i,), and in it a cylinder of sheet copper (0, Fig.
2), which is usually cleft clown one side.
Within the copper cyl inder is a porous pot of unglazed earthen ware containing dilute sulphuric acid. or pref erably a dilute solu tion of zinc sulphate (/.ISO,). The porous
pot contains also the zinc prism, Z. The object in using the unglazed pottery to separate the two solutions is to prevent their rapid ad mixture, while the ions readily pass through.
With sulphuric acid in the porous pot, the hydrogen ions are intercepted at the porous par tition by the copper sulphate. The positive cop per ions then migrate toward the copper elec trode, and are there deposited as metallic copper. The SO, ions go to the zinc electrode with their negative charges, as in the case of the simple voltaic cell.
When the zinc is immersed in dilute zinc sul phate, both sulphates undergo partial dissocia tion, and no hydrogen inns are then present. Only zinc and cop per ions travel tow ard the copper elec trode. The zinc ions are not deposited on the popper plate. be cause zinc in copper sulphate invariably replaces the copper.
forming ZnSO, in place of the CuS0,.
Zinc sulphate is formed at the nega tive. and metallic copper is deposited on the copper elec trode. At the same time there is a loss of copper sulphate cor responding exactly to the increase of zinc sulphate. The Daniel] cell is one of the most constant elements yet devised, for in it polarization by hydrogen is entirely avoided.