Page:The New International Encyclopædia 1st ed. v. 20.djvu/257

* VOLTAIC CELL. 209 VOLTAIC CELL. ically with one of theiiij is a voltaic cell or element. Several such cells joined tof^ether com- pose a batter}'. If the zinc and copjier plates are connected with a wire an electric current flows around in a circuit, from the copiicr to the zinc through the wire, and from the zinc to the cop- per Ihrouyh the liquid. The copper plate, or its ei[uivak'nt, is accordingly now called the positive electrode, or cathode, and the zinc plate the negative electrode, or anode. The modern theory of dissociation furnishes an explanation of the manner in which an electric current is conducted tlirough a liipiid. It is briefly as follows: When an alkali, a salt, or an acid, such as hydrochloric acid (IJCl), is dis- solved in water, some of the molecules, at lejist of ji_binary compound, split into two parts (II and CI, for example), one part having a positive elec- tric charge and the other a negative one. The tw'o parts of the dissociated substance wdth their electric charges are called ions. An electrolyte is a compound capable of such dissociation into two or more ions. It conducts electricity only by means of the migration of the ions resulting from the splitting in two of the molecules. The separated ions convey their charges with a slow and measurable velocity through the liquid. Elec- tro-jjositive ions, such as zinc and hydrogen, carry positive charges in one direction; electro- negative ions, such as chlorine and sulphion (SOj), carry negative charges in the opposite direction; and the sum of the two kinds of charges carried through the Jicjuid in a second is the measure of the current. The appearance of the hydrogen at the cathode only is thus ex- plained. See Electro-Chemistky. Local Actiok. When acid is used in a cell as the electrolyte, with commercial zinc, there is always evidence of chemical action which con- tributes nothing to the current, for it goes on when the circuit is open. This is known as local action. It is probably due to particles of carbon, iron, etc., in the zinc; these form with the zinc itself miniature voltaic cells, the currents flow- ing in short local circuits between the foreign particles and the zinc. For this reason the zinc is eaten away locally w'ith the formation of pits in its surface. The remedy for this local action was found in 1830 to be amalgamation of the zinc, that is, the formation on its surface of a mercury-zinc amalgam. The amalgam brings pure zinc to the surface, covers the foreign par- ticles, and, above all, forms a smooth surface to which a film of hydrogen adheres and protects it from chemical action, save when the circuit is closed. If tliis protecting film of hydrogen be re- moved by exhausting the air above the li<piid, the acid will attack the amalgamated plate as if it ere impure zinc. Hydrogen is found to afford a similar protection from chemical action in other cases. POLAKIZATION. If a simple voltaic cell is put in circuit with a galvanometer it may be ob- served that the current gradually diminishes. If 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 luider the term polarieafion. The hydrogen at the posifive electrode is the chief agent in decreasing the electromotive force of the Fig. 2. DANIELL CELI/. cell, by means of the negative or counter-electro- motive force which it produces. To prevent [)olarization means are adopted to replace the hydrogen ions with others, such as copper or mercury, which do not ])roduce polar- ization ; or (dse the positive eh'ctrode is sur- rounded with a chemical which furnishes oxygen or clilorine to unite with the hydrogen before it reaches the electrode. The Daniei.l Cell, invented by Professor Baniell, of London, in 1830, was the first suc- cessful attempt to prevent polarization by chem- ical methods. It con- si.sts of a glass jar containing a saturated solution of copper sul- jihate (CuSO.,), and in it a cylinder of sheet copper (C, Fig. 2), which is usually I left down one side. Uithin the copper cyl- inder is a porous pot of uiiglazcd earthen- ware containing dilute sulphuric acid, or pref- erably a dilute solu- tion of zinc sulphate (ZnSOi). 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 arc 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 ions are then present. Only zinc and cop- per ions travel tow- ard the copper elec- trode. The zinc ions are not deposited on the co])])er plate, be- cause zinc in copper sulphate invariably replaces the copper, fcu'ming ZuSO, in place of the CuSO,. Zinc sulphate is formed at the nega- tive, and metallii- copper is deposited on the copper elcc trode. At the same time there is a loss of copper sulphate cor- resixmding exactly to the increase of zinc sulphate. The Daniell cell is one of the most constant elements yet devised, for in it polarization by hj-drogen is entirely avoided. The CtRAvitt Cell is a modified Daniell (Fig. 3). The porous pot is omitted, the partial sep- aration of the liquids being secured by difference FjG. 3. GRAVITY CELL.