themth of an inch, the other th of an inch thick, and arranged them in coils in the manner that I have described (7). These were immersed in two glass jars, each of which contained nine ounces avoirdupois of water. A current of the mean quantity 1°.1 Q,* was then passed consecutively through both coils, and at the close of one hour I observed that the water in which the thin wire was immersed had gained 3°4, whilst the thick wire had produced only 1°3.

10. Now by direct experiment, I found that three feet of the thin wire could conduct exactly as well as eight feet of the thick wire; and hence it is evident that the resistances of two yards of each were in the ratio of 3-4 to 127, which approximates very closely to the ratio of the heating effects exhibited by the experiment.

11. Exp. 2.-1 now substituted a piece of iron wire 17th of an inch thick, and two yards long, for the thick copper wire used in Exp. 1, and placed each coil in half a pound of water. A current of 1°.25 Q was passed through both during one hour, when the augmentation of temperature caused by the iron was 6°, whilst that produced by the copper wire was 5°•5. In this case the resistances of the iron and copper wires was found to be in the ratio of 6 to 5·51.

12. Exp. 3.-A coil of copper wire were then compared with one of mercury, which was accomplished by inclosing the latter in a bent glass tube. In this way I had immersed each in half a pound of water, 114 feet of copper wire of an inch thick, and 22 inches of mercury 0.065 of an inch in diameter. At the close of one hour, during which the same current of electricity was passed through both the former had caused a rise of temperature of 4° 4, the latter of 2°.9. The resistances were found by a careful experiment to be in the ratio of 4.4 to 3.

13. Other trials were made with results of precisely the same character; they all conspire to confirm the fact, that when a given quantity of voltaic electricity is passed through a metallic conductor for a given length of time, the quantity of heat evolved by it is always proportional to the resistance which it presents, whatever may be the length, thickness, shape, or kind of that metallic conductor.

14. On considering the above law, I thought that the effect produced by the increase of the intensity of the electric current would be as the square of that element; for it is evident that in that case the resistance would be augmented in a double ratio, arising from the increase of the quantity of electricity passed in a given time, and also from the increase of the velocity of the same. We shall immediately see that this view is actually sustained by experiment.

15. I took the coil of copper wire used in Exp. 3. and have found the different quantities of heat gained by half a pound of water in

• I place Q at the end of my degrees, to distinguish them from those of the graduated card.

+ Mr. Harris, and others, have proved this law very satisfactorily, using common electricity.

which it was immersed, by the passage of electricities of different degrees of tension. My results are arranged in the following table: :

16. The differences between the numbers in columns three and five, and those in columns four and six, are very inconsiderable, taking into account the nature of the experiments, and are principally owing to the difficulty which exists in keeping the air of the room in the same state of quiet, of hygrometry, &c., during the different days on which the experiments were made. They are much less when a larger quantity of water is used, so as to reduce the cooling effects. -(28).

17. We see, therefore, when a current of voltaic electricity is propagated along a metallic conductor, the heat evolved in a given time is proportional to the resistance of the conductor multiplied by the square of the electric intensity.

18. The above law is of great importance. It teaches us the right use of those instruments which are intended to measure electric currents by the quantities of heat which they evolve. If such instruments be employed, (though in their present state they are far inferior in point of accuracy to many other forms of the galvanometer), it is obvious that the square roots of their indications are alone proportional to the intensities which they are intended to measure.

19. By another important application of the law, we are now enabled to compare the frictionalt and voltaic electricities, in such a

The experiments of De la Rive show that the calorific effect of the voltaic current increases in a much greater proportion than the simple ratio of the intensities. Ann. de Chimie, 1836, part i, p. 193. See also Peltier's results, Ann. de Chimie, 1836, part ii, p. 249.

The experiments of Brooke, Cuthbertson and others, prove that the quantity of wire melted by common electricity is as the square of that battery's charge. Harris, however, arrived at the conclusion, that the heating power of electricity is simply as the charge, Phil. Trans., 1834, p. 225. Of course the remark in the text is made on the presumption, that when the proper limitations are observed, the calorific effect of electricity is as the square of the charge of any given battery.

manner as to determine their elements by the quantity of heat which they evolve in passing along a given conductor; for if a certain quantity of voltaic electricity produce a certain degree of heat by passing along a given conductor, and if the same quantity of heat be generated by the discharge of a certain electrical battery along the same conductor, the product of the quantity and velocity of transfer of the voltaic electricity, will be equal to the product of the quantity and velocity of the frictional electricity, or Q V qv, whence Q V

=

v

CHAP. II.-Heat evolved during Electrolysis.

20. Under the above head, I shall now examine the heat produced in the cells of the battery, and when electrolytes are experiencing the action of the voltaic current. It has been my desire to render these experiments strictly comparable, both with themselves and with those of other philosophers. I have therefore taken care to apply the corrections which either specific heat, or other disturbing causes might require, and have by these means been able to express in every case, the total amount of evolved heat.

21. The first of these corrections, which I call Cor. A, arises from the difference between the mean temperature of the liquid used in an experiment, and that of the surrounding atmosphere. Its amount is determined by ascertaining the rapidity with which the temperature of the liquid is reduced at the end of each experiment.

22. The second correction (Cor. B) is for the specific heat of the liquids, and the vessels which contain them; and when the necessary data could not be found in the tables of specific heat, I have supplied them from my own experiments. The vessels were white earthenware jars, 4 inches deep, and 44 inches in diameter: their caloric was one-twelfth of that contanied by two pounds of water, to which capacity I have reduced all my subsequent results.

23. As resistance to conduction is the sole cause of the heat produced in the connecting wire of the voltaic battery, it was natural to expect that it would act an important part in this second class of phenomena also. It was important, therefore, to begin by determining the amount of heat evolved by that quantity of conducting metal which I found it convenient to adopt as a standard of resistance.

24. Ten feet of copper wire, 0.024 of an inch thick, were formed into a coil in the manner described in (7); its resistance to conduction was called unity. Three experiments were made in order to ascertain its heating power.

25. 1st. A jar was filled with two pounds of water, and a current which produced a mean deviation of the needle of the galvanometer (3) equal to 57° 2°54 Q of current electricity, was urged through the coil for twenty-seven minutes, by means of a zinc-iron*

Whenever an iron battery was used, it was of course placed at a distance from the galvanometer sufficiently great to render its action on the needle altogether inappreciable.

battery of ten pairs. The heat thus acquired by the water, after Cor. A, and that part of Cor. B which relates to the caloric of the jar, had been applied, was 6°.22.

26. 2nd. The battery was now charged with a weaker solution of sulphuric acid. In this case it passed the mean current 2°.085 Q during forty-five minutes. The heat thus produced, when corrected, was 7° 04.

27. 3rd. A battery of five pairs (three of which had platinized silver; one silver, and one copper, for their negative plates,) passed the mean current 1°.88 Q during one hour, in which time 7°47 were generated.

28. When the first two experiments are reduced, in order to compare them with the third, we have, in accordance with the prin(1.88)2 60' × 6°.22 = 7°57, (2.54)2 27

ciples laid down in (17),

70.577°.63 + 7°:37

3

=7°.56, the mean and total quantity of heat produced per hour by the passage of 10.88 Q of current electricity, against the unit of resistance.

29. Before I proceed to give an account of some experiments on heat evolved in the cells of voltaic pairs, it is important to observe that every kind of action not essentially electrolytic must be eliminated. For instance, the dissolution of metallic oxides in acid menstrua, which has been proved by Dr. Faraday to be no cause of the current, is the occasion of a very considerable quantity of heat, which, if not accounted for in the experiments, would altogether disturb the results. I have taken the oxide of zinc, prepared either by igniting the nitrate, or by burning the metal, and have repeatedly dissolved it in sulphuric acid of various specific gravities; and on taking the mean of many experiments, none of which differed materially from the rest, I have found that the total corrected heat produced by the dissolution of 100 grains of the oxide of zinc in sulphuric acid, is able to raise two pounds of water 3°.44.

30. Exp. 1.-I constructed a single voltaic pair, consisting of thin plates of amalgamated zinc and platinized silver (Mr. Smee's arrangement): the plates were two inches broad, and were kept one inch asunder by means of a piece of wood, to the opposite sides of which they were bound with string: to the top of each plate, a thick copper wire formed a good metallic connexion, by means of a brass clamp. The voltaic pair, thus prepared, was immersed in two pounds of sulphuric acid, sp. gr. 1137, contained by one of the earthenware jars (22). The arrangement is represented by Fig. 3.

31. When the circuit was completed so as to present to the current the total metallic resistance 0.06, the galvanometer stood at 49° 1°.84 Q; and at 17100°453 Q, when the total metallic resistance was increased to 1.16 by the addition to the circuit of ten feet of thin copper wire. Hence, according to the 1.84 0.453

principles laid down by Ohm,

=

Fig. 3.

+1-16 r+006' from which r, the resistance of the voltaic pair, 0-299. Immediately after this trial, the temperature of the liquid being exactly 49°, and that of the air 50°-2, the circuit was completed for one hour, during which the needle first advanced a little from 50°, and then declined to 46°, the average* deviation was 48° 44' = 18 Q. The temperature of the liquid was then 53°-7, indicating a rise of 4°.7.

1.59 0.382 '+1.16 r'+0.06

; whence r', the resis

Another trial now gave tance of the pair at the close of the experiment, 0.288: the mean resistance of the pair was therefore 0.293.

32. Now in order to obtain the total amount of heat evolved by the pair, reduced to the capacity of two pounds of water, we have 4°.7+0°4 (on account of Cor. A (21)) and 0°.5 (on account of Cor. B (22)) = 4°.6. The correction due to the dissolution of oxide

3°.44

of zinc is found by multiplying its quantity by (see (29)); the

100

quantity of the oxide being obtained by multiplying the equivalent of oxide of zinc by the mean quantity of current electricity. We 3°.44 100

have then 4 0.3 x 1.8 x

2°5: this, when substracted

from 4°.6, leaves 2°.1, the correct voltaic heat.

33. Assuming in this case, as well as in that of a metallic conductor, that the heat evolved is proportional to the resistance multiplied by the square of the electric intensity, we have, from the data

(1-8)2 (1.88)2

in (28) and (31), × 0·293 × 7o·56 = 2°03, which is very near 2°.1, the heat deduced from experiment.

34. Exp. 2.-I now constructed another pair, consisting of plates precisely similar to those used in Exp. 1, but half an inch only asunder it was also immersed in two pounds of sulphuric acid, sp. gr. 1137. The circuit was closed for one hour, during which the needle of the galvanometer advanced gradually from 4710, to 50°, the mean deviation being 49° 35′ = 1°.84 Q. The liquid had then gained 48: this, +0°-1 (for Cor. A) and -0°-5 (for Cor. B), =

During each experiment the deflections of the needle were noted at intervals of five minutes, or less. From thence I deduce my averages.