4°4. The heat due to the dissolution of oxide of zinc is in this 3°.44 = case 4 0-3 x 1.84 x 2°.55, which, when subtracted from 4°.4, leaves the correct voltaic heat 1°.85. 35. The resistance of the pair was ascertained in every other instance, at the beginning and at the experiment. and The equations thus obtained were = 0.446 this, as in end of the 1.714 r+ 1·16 = 0.432 1.91 ; whence r = 0·311, and r + 0.06, r' + 1.16 g = 0.275 the mean resistance was therefore 0.293. Now, calculating as before (33), on the basis of the heat produced by the passage of electricity against the standard of resistance, we have X 0·293 x 7°.56 (1.84)2 (1.88)2 2°-12. 36. Exp. 3.-I formed another pair on Mr. Smee's plan; it was similar to the last, with the exception that the plates were only one inch broad. When the circuit was closed, a current of the mean intensity 1°46 Q passed through the apparatus during one half hour. The heat thereby produced, when corrected, and reduced on account of the dissolution of oxide of zinc, was 0.84. 37. In this instance the mean resistance was 0-32; whence, by a calculation precisely similar to those given under Exps. 1 and 2, we have the theoretical amount of heat =0°.74. 38. The three instances above given, are specimens taken from a number of experiments with the platinized silver pairs. The mean of the eight unexceptionable experiments which I have made with them, gives 2008 of actual, and 2°-13 of theoretical heat, and not one of the individual experiments presented a greater difference between real and calculated heat, than Exp. 2. 39. Exp. 4.-A plate of copper, four inches broad, was bent about a plate of amalgamated zinc three inches and a half broad, so as to form a pair of Wollaston's double battery. It was placed in a jar containing two pounds of dilute sulphuric acid. In this instance, the total voltaic heat that was generated was 1°2, the calculated result being 100 only. Repeated experiments with the copper pairs gave similar results, the real heat being invariably somewhat superior to that which the doctrine of resistances would demand. The cause of this I have found in a slight local action, which it is almost impossible to avoid in the common copper battery. 40. Exp. 5.-I now constructed a single pair on Mr. Grove's plan. The platinum, two inches broad, was immersed in an ounce and a half of strong nitric acid contained by a 4-inch pipe-clay cell; the amalgamated zinc plate, also two inches broad, was immersed (at the distance of an inch and a half from the platinum) in thirty ounces of sulphuric acid, sp. gr. 1156. The whole was contained by one of the jars (22). 41. A trial, made first as usual, in order to ascertain the resistance 4.4 = 0.816 of the pair, gave,+2.26 = 0·441. r+ 9.06; whence r As soon as the slight heat acquired during the above trial was equably diffused through the apparatus, the thermometer placed in the dilute suphuric acid stood at 51°.95, the temperature of the air being 52° 4. The circuit was then immediately closed for ten minutes, during which time the needle of the galvanometer advanced steadily from 68° 40′ to 71° 20′; the mean deviation being 70° 9′ 4°.77 Q. As soon as the heat thus generated was equably diffused*, the thermometer immersed in the dilute sulphuric acid stood at 56°7, indicating a rise of 4°.75. Another trial now gave 0.91 5.14 = r' + 2·26 r' + 0·06' whencer 0413. The mean resis tance of the pair was therefore 0-427. 42. 4°.75 + 0°·1 (for Cor. A), and 0°.4 (for Cor. B, which in this case includes the capacity for heat of the porous cell) = 4°·45. The heat generated by the dissolution of oxide of zinc was in this 3°.44 10 case 40.3 x 4.77 X X = 1°1, which, subtracted from 100 60' 4°.45, leaves the correct voltaic heat 3°.35. 43. The theoretical result is 3°.46. (4-77)2 × 0·427 x 7°56 × 10' 60 44. Exp. 6 was made with a pair in every respect similiar to the last: the circuit, however, was completed by means of a thin copper wire, in order to reduce the intensity of the current. At the end of one hour, during which the needle of the galvanometer advanced gradually from 41° to 42°, the correct voltaic heat that was generated was 1°.7. The theoretical result was 1°.82. 45. I was desirous of knowing how far the same principles would apply to the heat generated in Professor Daniell's constant battery. But in this battery considerable cold is produced, in consequence of separation of oxide of copper from the sulphuric acid to which it is combined. This is altogether a secondary effect, and should be eliminated as decidedly as the heat produced by the dissolution of oxide of zinc. I have not yet been able to obtain accurate data for the correction thus needed, and shall therefore content myself with remarking, that my result with Mr. Daniell's arrangements are, as far as they go, quite consistent with the theory of resistances. 46. Experiments, such as I have related, were varied in many ways; and sometimes a number of pairs were arranged so as to form a battery. Still the results were similar, and established the fact, that the heat which is generated in a given time in any pair, by true voltaic action, is proportional to the resistance to conduction of that pair, multiplied by the square of the intensity of the current. By gently stirring the dilute sulphuric acid with a feather, so as to bring every part in successive contact with the porous cell during two minutes. 47. I now made some experiments on the heat consequent to the passage of voltaic electricity through electrolytes. of 48. Exp. 7.-Two pieces of platinum foil, each of which was an inch long, and a quarter of an inch broad, were hermetically sealed into the ends of two pieces of glass tubing: within these tubes, pieces copper wire were metallically connected with the platinum; these, when the apparatus was in action, terminated in mercury cups. The tubes thus prepared were bound together by thread, so as to keep the pieces of platinum foil at the constant distance of half an inch asunder. This apparatus was immersed in two pounds of dilute sulphuric acid, sp. gr. 1154, contained in one of the jars (22). 49. A battery of twenty (four inch) double iron zinc plates, was then placed, with its divided troughs (which were charged with a pretty strong solution of sulphuric acid), at a distance from the galvanometer sufficiently great to obviate any disturbing effect on the needle. To the electrodes of this battery thick copper wires were secured, so that by means of one of them connexion could be made to the galvanometer, and by means of the other, to the decomposing cell. In Fig. 4, A represents the battery, G the galvanometer, and Fig. 4. A G E the decomposing apparatus (48). 50. In order to ascertain the resistances of the battery or of the cell, I provided several coils* of silked copper wire, the resistances of which had been determined by careful experiments. When these were traversed by the current, they were placed in such a position as to prevent any action on the needle, and at the same time they were kept under water, in order to prevent them from becoming hot, which would have had the effect of increasing their resistances. 51. When everything was duly prepared, the battery was placed in its troughs, and the current from it was urged through the galvanometer and each of the three of the coils, which were placed in succession at E (the decomposing apparatus having been removed). The resistances of these coils, were 4.4, 5.5, and 7·7, and the currents which they allowed to pass were 1o·88 Q, 1°·65 Q, and 1°.29 Q. 52. The decomposing apparatus was now replaced, and the proper connexions being made, electrolytic decomposition was allowed to proceed during twenty minutes, in which time the needle of the galvanometer gradually declined from 55° to 48°, the mean current Two of these coils had been previously employed (31, 41), &c., in ascertaining the resistances of the voltaic pairs: the resistance 0.06 was that of the galvanometer and connecting wires. being 10.9 Q. The temperature of the liquid had now advanced from 46°.6 to 53°.95, indicating an increase of 7°.35. The temperature of the surrounding atmosphere was 46°.4. 53. The decomposing cell was now removed again, and the several coils, of which the resistances were, as before, 4·4, 5·5. and 7·7, were successively put in its place. The battery now urged through them, 1°.73 Q, 1°48 Q, and 1°.22 Q. 54. In this case 7°35 + 0°·55 (for Cor. A) and 0°.46 (for Cor. B)=7°-26, the heat which was generated in the decomposing jar. 1°.88 +1°.73 55. The mean intensity of the current when passing through the coil of which the resistance was 44, was = 1°.805 Q, but 1°.9 Q when it passed through the decomposing cell. Hence (4.4+3·15*) 1.805 of obstruction presented by the decomposing cell. 56. Now we must remember, that when the electric current was passing through the coils, it was urged by the whole intensity of the battery; but that in the case of the decomposing cell, a part of the intensity of the zinc-iron battery, equal (as I have found by experiment) to 3 pairs, or to one-sixth part of the whole, is occupied solely in overcoming the resistance to electrolyzation of water in the decomposing cell. In order therefore to deduce the true resis4.02+3.15 tance to conduction, we must subtract from the obstruc6 tion 4.02; and thus we have 2.83, the true resistance to conduction of the decomposing cell. 57. The latter part of this process is difficult to express clearly, I have therefore drawn a figure to illustrate it. Suppose that in Fig. 5, 6 represents the intensity of the battery; the line R 3.15 the resistance of the battery and the connecting wires; and the remainder of the line A B, or 4·02 W, the resistance of wire. I have shown (55) that the current 1°.9 Q would pass against the resistance A B. But we know that 10.9 Q was also passed when the cell and the battery formed the sole opposition (52), and that on account of = R'+4.4' whence R 2·81 and R' = 3.49: the mean resistance of the = battery and connecting wires was therefore 3.15. + Faraday's Experimental Researches (1007). the resistance to electrolyzation, the virtual battery intensity was then one-sixth less, and hence that only five-sixths of the resistance represented by A B could have been opposed in this case, in order to the passage of the same current. Draw, therefore, another line, C D, one-sixth less than A B, and it will represent this resistance; from which, on subtracting R 3·15, we have r 2·83, the true resistance to conduction of the decomposing apparatus. 58. From (28), and the data above given, we have 20' 2.83 x 7°56 × = 70.29, the theoretical result. (1.9)2 (1.88)2 X 59. I made three other experiments with the same electrodes, and with the same battery. The results of these with those of the experiment just given at length, are as follows: 60. Exp. 11.-The mean current 0°-846 Q from a battery of ten zinc-iron pairs, was, by means of the same electrodes, sent through two pounds of dilute sulphuric acid for half an hour, during which the correct heat that was generated was 3°·09. 61. In order to find the true resistance to conduction of the decomposing cell, it was necessary to remember that in this instance one-third of the intensity of the ten pairs was expended in overcoming the resistance to electrolyzation of the water. With this exception the calculations were made precisely as before, and gave 3.76, the resistance of the cell; whence we have the theoretical heat 2°.88. 62. I now dismissed the narrow electrodes, and substituted for them two pieces of platinum foil, dipping to the bottom of the liquid; they were one inch apart, and each presented to the dilute sulphuric acid a surface of seven square inches. In this case I used twenty pairs of zinc-iron plates arranged in a series of ten. 63. The mean of six experiments with this apparatus gave 4°.42 of real, and 4°.13 of theoretical heat. I have no doubt that the difference is principally occasioned by the formation of the deutoxide of hydrogen, which is known to occur to a considerable extent when oxygen is evolved from an extended surface. Of this we have another instance in the following experiment. 64. Exp. 12.-Using the same electrodes, and a battery of ten zinc-iron pairs, I new passed a current of the mean intensity 1°.08 Q, through two pounds of dilute nitric acid, sp. gr. 1047, for half an hour. The heat that was thus generated, when properly corrected, was 3°. |