VOLTAIC CELL or BATTERY. As early as 1700 Johann Sulzer announced to the Berlin academy of Science the discovery that a peculiar bitter taste is perceived when two metals are placed on the tongue and touched at their edges. The significance of this observation was not ap preciated till Volta had described his 'crown of cups' in 1800. The two metals, copper and zinc, for example, and the saline saliva compose, as is now known, a voltaic couple or cell.
While pursuing his experiments to explain Galvani's discovery (see GALVANIC BATTERY), Volta, of the University of Pavia, was led to the invention of the voltaic pile, where two dis similar plates, as zinc and copper, were placed the one on the other. Then followed a piece of cloth moistened with a weak solution of common salt. The cloth was surmounted by another pair of metal plates in the same order as the first, and so on, each pair being separated from ad jacent ones by moist cloth.
Volta's `couronne de tassel,' or 'crown of cups,' was in principle essentially the same as a single couple of the pile. A series of cups contained brine or dilute acid; into the liquid dipped metallic strips, half zinc and half copper. The zinc end of a strip dipped into one cup. and the copper end into the next one. An electric current was taken from the terminal wires of the series. Such was the first voltaic cell on record. In the words of Sir Humphry Davy, "the voltaic battery was an alarm bell to experimenters in every part of Europe." On April 30, 1800, Nicholson and Carlisle discovered that water could be decom posed by the electric current, and this important discovery was followed by Davy's production of the electric arc light between two pencils of car bon, the source of the current being a large vol taic battery; and in IS07 he accomplished the electrolysis of the fixed alkalis. potash and soda, and discovered the corresponding metallic ele ments, potassium and sodium.
Volta's battery will be best understood by con sidering a single simple cell. if a plate of zinc be placed in sulphuric acid, diluted with about twenty times its vol ume of water, bubbles of hydrogen will at first collect on the zinc, but visible chem ical action will soon cease. If now a, plate of copper be placed in the same solution no change will he ob served so long as the two metals are kept out of eontaet; but as soon as they are made to touch or are joined by wires (Vig.
1 ), vigorous ehemical action sets in, the zinc wastes away, and hydrogen gas is freely lib erated from the surface of the copper plate instead of the zinc. This ehemical action goes on only so long as the two plates are electrically connect ed. Such a combination of two con duetors, immersed in a compound liquid, called an electrolyte, which is capable of reacting chcm• ically with one of them, is a voltaic cell or element. Several such yells joined together com pose a battery. If the zinc and copper plates are connected with a wire an electric current flows around in a circuit, from the to 1.1c zinc through the wire, and from the zinc: to the cop per through the liquid. The copper plate. or its equivalent, is accordingly now Balled the positive electrode. or cathode, :nil the zinc: plate the negative electrode, or anode.
The modern theory of diss(Wiation furnishes an explanation of the manlier in which an electrie current is conducted through a liquid. It is briefly as follows: When an alkali, a salt, or an acid, such as hydrochloric acid (11e1 ), is dis solved in water, some of the molecules, at least of a binary compound, split into two parts (fi and ('I, for example), one part having a positive elec tric charge and the other a negative one. The two parts of the dissociated substance with 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-positive ions, such as zinc and hydrogen, carry positive elmrges in one direction; I•lectro negative ions, such as chlorine and sulphion (SO,), carry negative charges in the opposite direction: and the sum of the two kinds of charges carried through the liquid in a. second is the measure of the current. The appearance of the hydrogen at the cathode only is thus ex plained. See ELECTRO-CHEMISTRY.