Page:Encyclopædia Britannica, Ninth Edition, v. 8.djvu/99

Rh ELECTROMOTIVE FOKCE.J ELEOTRIGIT V Jlama tn in Although the polarization by gas deposits has absorbed so much of the attention of physicists, it is by no means a solitary instance. The phenomenon is universal. It ap pears even with zinc plates in zinc sulphate, and copper plates in copper sulphate. The nearest approach to unpo- larizable electrodes is the case of amalgamated zinc plates in zinc sulphate, originally discovered by Du Bois Rey mond. When the sulphate solution is neutral, the polari zation, as may be shown by immersing a large number oi plates in series in the solution, is extremely small. For an account of polarization at the surface of two liquids observed by Du Bois Reymond, and other kindred matters, and for many other facts which we have passed over in silence, the reader may consult Wiedemann s Gal- vanlsmus. Some account will be found in the article ELECTROLYSIS of the remarkable phenomenon of the &quot; passivity of iron, and of the powerful polarization arising from the formation of superoxides, on which depends the action of the secondary pile of Plante.&quot; Application of f/te Laws of Energy to the Voltaic Circuit. In the classical series of researches by which Joule laid the foundations of the laws of energy, a considerable share of attention is devoted to the energetics of the electric current. Guided by the great idea which he was gradually developing, Joule made experimental determinations of the amount of energy of various kinds evolved in the electric circuit. We have already seen how he measured the quantity of heat developed in a metallic conductor, and in an electrolyte. 2 This quantity was found to vary as the product of the resistance of the conductor into the square of the current strength, account being taken of disturbances at the electrodes in the case of electrolytes. These disturbances were considered in the first memoir and allowed for. The accuracy of the view taken of them, to which Joule was led by the opinion of Faraday, that the solution of the oxide in the voltaic cell had no active -share in producing the electric current, was justly ques tioned, implicitly by Sir Wm. Thomson 3 in 1851, and ex plicitly by Bosscha 4 in 1859. In a later memoir, however/ Joule made a direct experi mental investigation of these secondary effects, and shows how they can be accounted for. His results have not been shaken by subsequent investigators ; and the general con clusions to be drawn from them are not in the least affected by the theory of secondary action, which is sug gested in the paper. These, so far as we are now con cerned with them, are as follows : &quot; 1st. In an electrolytic cell there are three distinct obstacles to the voltaic current. The first is resistance to conduction; the second is resistance to electrolysis without chemical change 1 [arising simply from the presence of 1 Tait, Phil. Mag., 1869. This method is in some respects one of me best for measurements of the kind Phil. Mag., 1841. 3 PUL x 1851 (2) 55i &quot;Offff. Ann., cviii. p. 319. 6 m 6 ^ 1 Ll. 1 atld Phii&amp;gt; Soc - Manchester, 2d ser. vii., 1843. IMS resistance is, in more modern language, an &quot; opposing elec tromotive force. . 7 The meaning of &quot; without chemical change&quot; will be seen directly. I ilarization of various Metals measured irith Ttwmsori s Quadrant Electrometer. 1 S Oxygen Plate. Hydrogen Plate. Polarization. No. of Celli in Polarizing Battery. Freshly ignited Pt. Pt 1-64-2 -30 1-8 Ft Pd 1-50-1-85 1-4 Pd Pt 1-60-1-91 1-4 Pt Fe 2-16 3 Fe Pt

3 Fe Fe

3 Al Al 1-09-5-20 1-6 chemical repulsion] ; 8 and the third is resistance to electro lysis, accompanied by chemical changes. &quot; 2d. By the first of these (the resistance to conduction) heat is evolved exactly as it is by a wire, according to the resistance and the square of the current; and it is thus that a part of the heat belonging to the chemical actions of the battery is evolved. By the second a reaction on the intensity 1 * of the battery occurs, and wherever it exists heat is evolved exactly equivalent to the loss of heating power in the battery arising from its diminished intensity. But the third resistance differs from the second, inasmuch aa the heat due to its reaction is rendered latent, and thus lost to the circuit. &quot; 3d. Hence it is that, however we arrange the voltaic apparatus, and whatever cells of electrolysis we include in the circuit, the whole caloric of the circuit is exactly accounted for by the whole of the chemical changes. &quot; 4th. As was discovered by Faraday, the quantity of current electricity 10 depends upon the number of atoms which suffer electrolysis in each cell ; and the intensity depends on the sum of chemical affinities. Now both the mechanical and heating powers of a current are (per equiva lent of electrolysis in any one of the battery cells) propor tional to its intensity. Therefore the mechanical and heating powers of the current are proportional to each other. &quot; 5th. The magnetic electrical machine enables us to con vert mechanical power into heat by means of the electric currents which are induced by it ; and I have little doubt that, by interposing an electromagnetic engine in the circuit of a battery, a diminution of the heat evolved per equiva lent of chemical change would be the consequence, and this in proportion to the mechanical powers obtained.&quot; 11 The above statement of Joule s contains, in a form which seems to us neither ambiguous nor obscure, 12 an exposition of the leading experimental principles of the energetics of the electric circuit. Besides the papers of Joule we have mentioned, two others on the electrical origin of the heat of chemical combination ought to be read in connection with this subject. 13 The now famous tract of Helmholtz, &quot; Ueber die Erhaltung der Kraft,&quot; which appeared in 1847, shortly after these papers of Joule, did much, by its able statement of the issues, to advance this branch of electrical science, and should be consulted by every thorough student. An extremely important contribution to the experimental Fav evidence for the law of energy in the case of electric cur- re - rents was furnished by the researches of Fav re. 14 He uses searcLeiJ - a calorimeter, which is virtually a mercury thermometer with an enormous bulb, into which are inserted a number of test-tube shaped vessels all opening outwards. When a heated body is placed in one of these vessels its heat is quickly communicated to the mercury in the calorimeter, and the amount of heat thus communicated is measured by the expansion of the mercury, which is measured a.s usual by noting the displacement along a capillary tube, [nto one of the recesses of the bulb of this calorimeter containing a quantity of dilute sulphuric acid was intro duced 33 grm. of granulated zinc. The heat evolved during its dissolution was 18682 units (gramme-degrees C.). Five of the recesses were then furnished with dilute sul- ihuric acid of the same strength as before, and into them were put five elements of Sinee (amalgamated zinc and 8 The brackets here are ours ; they contain Joule s theoretical view with which we are not now concerned. 9 Tn modern phrase, &quot;electromotive force.&quot; That is, current strength. 11 This he experimentally verified, Phil. Mag., 1843. 12 Cf. Verdet, Theorie Mecanique de la Chvlew, 327. 13 Phil. Mag., 1842 (1), and 1843 (1). u .4r&amp;gt;n. df &amp;lt;J!tim. e.t de Phys., 1854. virr. 12