Page:Encyclopædia Britannica, Ninth Edition, v. 8.djvu/97

Rh ELECTROMOTIVE FORCE.] ELECTRICITY 87 posing electromotive force arising from the alteration of the contacts. The former was the explanation adopted in the earlier researches of Poggendorff and Fechner ; but there can be no doubt about the existence of the latter effect, and Lenz showed that it was sufficient to account for the facts observed. It has been usual, therefore, to neglect the transition resistance altogether in the great majority of cases. It is certain, however, that it really exists in some instances. Consider the case of two electrolytic cells containing concentrated sulphuric acid, the electrodes being in the one copper plates, in the other platinum plates. Either of these cells inserted into a voltaic current will weaken the current, but for different reasons. In both cases hydrogen is freed at the negative electrode, and reduces sulphur from the strong acid, the effect of which is not great either way, for if the negative electrode be replaced by a fresh plate the weakening of the current remains. At the positive electrode oxygen is evolved, which oxidizes the copper in the one case and is deposited on the platinum in the other, in both cases replacing the positive electrode by a fresh plate will cause momentary increase in the current ; but the copper plate which served as positive electrode if tested against a fresh copper plate gives very little return current, whereas the positive plati num plate similarly tested gives a very powerful one. In the one cell the weakening of the current was due to the resistance of a crust of non-conducting copper oxide, in the other it was due to the contact force at the surface of the oxygenated platinum. 1 The polarization which arises from the deposition of gas on th e electrodes is, if we except the case where peroxides are formed, by far the most powerful form, and has been much studied ever since voltaic batteries were used. Ex periments like the one just quoted prove decisively that the electromotive force has its seat at the surface of the electrode itself, and is due to local alterations there. The certainty of this fact gives the study of the phenomenon great theoretical importance, since we may hope thereby to arrive at some idea of the nature of the contact force. It is also certain that in most cases each electrode con tributes separately to the whole electromotive force, for if we remove the polarized plates the adhering gas goes with them, and each plate is found to be electrically different when tested against a fresh or unpolarized plate. We n?ay also take measures to remove the deposited gas by washing the plates with water, potash, or nitric acid, or by igniting them ; and it is found that the more energetic the chemical agency thus employed the more thoroughly is the polarization destroyed. It seems clear, therefore, that it is the mere fact of the presence of the gas on the electrode in some form or other which causes the electrical difference. We may go still further and produce the phenomenon by depositing gas by means other than electrolytic. If a piece of platinum foil be immersed in hydrogen it absorbs the gas, as has been shown by Graham. A piece of foil thus treated is positive to a piece of freshly ignited foil when both are placed in dilute sulphuric acid. The same result is obtained by satu rating the liquid in the neighbourhood of the foil with hydrogen. 2 The activity thus conferred on the plate may be again destroyed by immersing it in chlorine or bromine, or even in oxygen, by igniting it, and so on. Similarly, if we dip a fresh piece of foil into chlorine or bromine, it will become negative to a fresh plate. The effect obtained by dipping the foil in oxygen in the ordinary state is very small, the oxygen deposited by electrolysis must therefore be in a different state to that condensed during mere im- Wiedemjum, Bd. i. 4of&amp;gt;. See Macaluso s experiments, &quot;Wiedemann, Nachtriiyc, 53. mersion in the gas. This is probably due to the fact that electrolytically generated oxygen contains ozone (see art. ELECTROLYSIS ); and in accordance with this we find that a platinum foil ozonized by being held in the electric brush proceeding from a charged conductor, or rubbed with phosphorus, is negative to a fresh plate in dilute sulphuric acid. The gas battery of Grove is a remarkable instance of the electro- Grove s motive properties of gas-coated metals. Two long glass tubes A gas and B are arranged in the two necks of a &quot;Woulfe s bottle. Tlie battery, upper ends of the tubes are closed, but pierced by two platinum wires, to which are fastened two long strips of platinum foil (which are sometimes platinized) 3 reaching to very near the lower ends of the tubes. The bottle and part of the tubes are filled with some liquid, say dilute sulphuric acid, and hydrogen introduced into B and oxygen into A. This may be very conveniently done by send ing an electric current from A to B and decomposing the dilute acid, but it may be done in other ways as well. This arrangement has an electromotive force comparable with that of a Daniell s cell (see below, p. 88), and if the original volume of hydrogen in B be twice that of the oxygen in A it will continue to send the current through a closed circuit, the gas gradually disappearing in the tubes until none is left, when the current stops. These gas elements may be connected up in series, &c., and used like ordinary battery cells. If the tube B be filled with ordinary hydrogen and A with liquid only, a current in the same direction as before is observed ; but the liquid in A is decomposed and hydrogen evolved, which produces an opposing electromotive force and stops the current. If A contain oxygen and B fluid only, the current lasts for a very short time unless the oxygen contain ozone. This is in accordance with what we have already seen. Grove * lias given an electromotive series for the different gases and metals as follows: Chlorine. Bromine. Iodine. Nitric oxide. Carbonic acid. Nitrogen. Metals which do Alcohol. not decompose Sulphur. water. Phosphorus. Camphor. Carbonic oxide. Volatile oils. Hydrogen. Olofiant gas. lletals which de- Ether, compose water. In this series any member is positive to any preceding member. We have already remarked that the polarization in many Electro- cases comes on very rapidly. In cases where the electro- lytic motive force of the battery is not sufficient to produce a P nvec&amp;lt; continuous evolution of gas, the current after the first rush dies away very rapidly. There are cases, however, in which a small current continues to flow for a very long time. Such currents are not accompanied by any visible evolution of gas, and it is clear that they could not be so accom panied, for, if the electromotive force of the battery be under a certain limit, the amount of chemical energy absorbed by the current per unit of time is less than the intrinsic energy of the ions liberated in a unit of time. It was originally supposed, therefore, that, besides this electrolytic conduction proper, fluids conducted to a slight extent like metals. But Helmholtz 5 has shown that no such assumption is necessary, and has pointed out that when the fluid holds gas in solution a sort of electrolytic conduction may take place which involves, it is true, libera tion of the ions, or at least of an ion, but so that the final result does not imply absorption of more energy than the buttery can furnish per unit of time in accordance with Faraday s law of electrolytic conduction. Suppose, for instance, that the dilute sulphuric acid in an ordinary voltameter held II in solution. &quot;When the current passes, appears at the anode and unites with the H in solution to form water; a corresponding quantity of H is thereby liberated at the cathode, which is either absorbed by the platinum electrode or diffuses to wards the anode, to combine in its turn with the appearing oxygen. It is obvious that the liberation of the ion in such a case docs not involve absorption of energy to the extent necessary when both H and O are disengaged from water. A current might therefore be kept up under such circumstances for a long time by an clectro- 3 This is best accomplished by washing the foils in hot nitric acid, and then using them to decompose a solution of platinic chloride with the current of two cells of Grove.
 * Phil. Trans., 1845. B Pogg. Ann., 1873.