ON THE WESTON C E L L AS A T R A N S I T I O N C E L L A N D AS -4 STANDARD O F E L E C T R O J I O T I V E FORCE, WITH A DETERMINATION O F THE R A T I O T O THE CLARK C E L L
BY H. T. BARNES
Kohnstamm and CohenI have recently made a careful study of the inversion point that occurs in the temperature curve of cadmium sulphate. They have shown conclusively by solubility determinations, by a transition cell of the second type and by the dilatometer, that this change takes place in the neighborhood of 15' C. Jaeger and Wachsniuth,z in their paper on the cadmium cell, noticed this change as an irregularity in their cells, but otherwise did not give it further consideration. As yet it appears, so far as the author is aware, that no experiments have been published where the discontinuity is shown by a change in the E. M. F. temperature curve of a Weston cell. No idea can be obtained from the experiments of Jaeger and Wachsmuth as to the position of this important transition point, on account of the method they used of determining the temperature change, but it is a matter of very little difficulty to employ the Weston cell as a transition cell of the fourth type, in a manner similar to the Clark cell.3 It is proposed in the present paper to describe dome experiments on this point, which, although carried out before the paper was published by Kohnstamin and Cohen, stand in good accord with their results. T h e primary object of the work was not so much to deWied. Ann. 65, 344 (1898). "bid. 59, 575 (1896). ,i Jo~ir.Phys. Chem. 4, I (1900).
340
P
H. 2: Bavizes
termine the temperature change of the cell as to prepare a set of Weston cadmium standards for comparison with the Clark cells in the possession of the laboratory. However certain experiments led the author to study more in detail the cell itself. It was soon seen that the temperature change was somewhat in excess of the formula proposed by Jaeger and Wachsmuth, and although perfectly consistent and regular above I j" C. for the types of cells used, yet showed such large discrepancies below 15" as to leave no doubt that they were inherent in the cells themselves and could not be attributed to any possible error of experiment. T h e question naturdly arose whether the dis: continuity in the curve was similar to that in the Clark cell at 39" C., but it was seen that the break was by no means so sharply defined. T h e great viscosity of the cadmium sulphate solution when nearly saturated, and the extreme slowness with which the last quantity of salt is dissolved, renders i t very probable that this would be so even if the break could be attributed to a change in hydration. From a careful examination of the salt above and below 15", Kohnstamni and Cohen were led to the idea that a change in crystalline form occurred: This would be difficult to show, but seems to explain more conclusively the existing data. Temperature change of the Weston cell given by previous observers From the experiments of Dearlove' on the Weston cadniiuiii cell the fact is very evident that the form of cell originally recommended by Weston and described in his patent specifications, was very irregular and uncertain. From the value 0.01 percent, as given by Weston for the temperature change per degree, Dearlove found values ranging to 0.003 percent. These he was unable to connect with properties of the cells, although he explained very well the cause for the irregularities between different cells. This uncertainty in the temperature coefficient was 50 marked that in concluding his remarks on the test of seven cells sent him by Dr. Muirhead, he says, " T h e temporary
' Electrician,
3 1 , 645 (1893).
On the Westoz Cell
341
coefficient of the above cells between 2 . 5 " C. and 2 2 " C. was found to be 0.008 percent." I I n the extensive experiments of Jaeger and Wachsmuth they do not attempt to explain the irregularities in the cells used by Dearlove, although they appear to have been aware of his results. Their experiments comprise comparisons of several cells set up in the H-form with different chemicals, and strengths of amalgam. They determine the temperature change of the E. M.F. by measuring the difference between six cells in paraffin baths a t different temperatures, but do not attempt to carry the cells through a temperature cycle. They study the effect of impurities in the ingredients of the cell and show the portable nature of the cell by comparing with a standard before and after making a journey of 600 Kni. T h e agreement between the calculated and observed values of the temperature change is most satisfactory, seldom exceeding 0.01 millivolt. At the same time, the range is not large, only in three instances being as much as 18". Three experiiiients only were made as high as 26" and the greatest range at this limit was between a cell at 17.8" C. and one at 25.9" C. Most of the tests were made, however, below 18". T h e temperature forniula deduced from these tests reads in millivolts.
E,=E,,
-0.038
(t-
20')
-
0.0006j
(t-20')'.
T h e authors, however, remark Diese Formel gilt nach ihrer Ableicnng zwischen 0" und 2 j 0 , doch hahen sFatere Reobachtungen gezeigt, dass eiiiige Eleniente sich zwixhen 0" und etwa 5" in der Weise unregelniassig verhalten, dass ihre electromotorische Kraft bedeutend (etwa f, V.) grosser ist, als diejenige der anderen Elemente. Der Rereich der obigeii Formel ist daher vorlaufig auf die Temperaturen zwischen 25" bis oberhalb j" zu beschranken."z I n a footnote given in the paper of Kohnstamm and Cohen, authorized by Prof. Jaeger, it appears that 5" should read 15". In Lhis case the remarkable
..
'1. c. p. 647. loc. cit. p. j83
2
I
H. T. Barizes
342
agreement of the observations of Jaeger and Wachsmuch extending below 15’ with their formula must be regarded as accidental. T h e temperature change per degree given by the above formula is almost exactly one-half of the value found by Dearlove and only about one-third of the value given by Weston. Present experiments T h e experiments about to be described were very much facilitated by the constant temperature water-baths and electrical and temperature measuring instruments employed in the author’s study of the Clark cell. T h e method of carrying out the experiments was also the same. In constructing the Weston cell in test-tube form, the B. 0. T. type of crystal cell was followed except that it was inverted. This is due to the necessity of using cadmium amalgam in place of a rod. This inverted cell has been already described in another place. I T h e following list comprises a brief description of the cells used in the present work, : Cd, A cell prepared similar in every respect to a B. 0. T.testtube crystal ” Clark cell, except in the use of moist crystals of cadmium sulphate in place of zinc sulphate, and an amalgamated cadmium rod in place of a zinc rod. T h e cadmium was carefully tested and found free from any trace of zinc. T h e cadmium sujphate was chemically pure and found neutral. It was not recrystallized or treated with mercurous sulphate. T h e paste was prepared by mixing pure washed mercurous sulphate with moist cadmium sulphate crystals. T h e positive electrode was a flattened platinuin wire, amalgamated. ((
.
Cd Similar in every respect to CdI except in the use of a cadmium rod not amalgamated. 1
Cd,Cd,Cd, Cells made in the inverted type with a cadmium .amalgain Electrician, 39, 68 (1897) ; 40, 165 (1897).
On the Westoiz Cell of
I
343
to 6 proportion, as recommended by Jaeger and Wachsmuth. Cd, H-form of cell made after type 111of Jager and Wachsmuth. Cda Cd,
Cells of the inverted type with a I to 6 amalgam. These cells differed from the other inverted cells only in having cadmium sulphate treated with mercurous sulphate before recrystallization. A comparison of these cells is given in Table I, and shows the divergence in millivolts from the mean.
Mar z m d
Cell
~
Mar 27th _____
to.02
....
. ... f0.06-
I
$0.01
$0.02
+0.03
-0.32 -0.04 $0.30
-0.04 sO.09
-0.34
0
0.32
Cells CdI and Cd2, being made with cadmium stick, were much higher than the amalgam cells. Cdl was 6.46 millivolts higher than Cd2 and no less than 44.0 millivolts higher than Cd3. A complete test of the temperature change was made on three of the cells included in the table over the range oo to 40°, between March 18th and 20th) which no doubt is the reason for the permanent change in the E. M. F. for Cd, and Cd,. T h e agreement of all the cells before this test is a sufficient proof that the divergence noted in these two cells is not to be ascribed to the manner of construction.
344
H. T. Barizes
TABLE I1 Temperature coefficient Weston Cadmium cell March ;9, 1897
I
I
Formula -0.086 (t-15")
$2.07
1.64 1.26 0.99 0.69 7.3 9.6 Ii.8 13.8 15.0 18.6 20. I
24.2 29.3 27.1
1
+4.33 3.68 3.30 3.02 2.64 I .90
0.21
0.14 0.23 0.07 0
-1.27
0.49 0.83 1.16 0.87
+3.64 3.08 2.59 2 33 1.93 0.86 0.32
.
1.17 0.55
0.17
0. I I
0.02
0
0
-0.22
-0.37
0.36
0.58 I .oo 1.38 0.98 1.66
0.73 1.20 1.11
e . . . e . . .
. ...
.... .... ... .... .... . ... I
0
-0.31 0.44 0.79 1.23 I .04
32.6 Put at 1.51 1 1.57 38. I 2.02 1.99 I5O 1.99 2.17 2.19 2.19 40.5 2.08 2.11 39.6 2.05 36. I I. 70 Put at 1.81 ...* 1.24 30.5 I 15O 1.33 I 0.92 *... j 0.92 25.7 T h e results of the experiments on the temperature change are contained in Table I1 and are plotted in Fig. I . T h e formula of Jaeger and Wachsmuth (J. & W) is also shown for comparison. I t appears that these cells follow within the limits of experimental error a linear formula I
....
e . . .
~
* . . e
....
E,=E,,---o.086 i t -
15O),
which agrees almost exactly with the mean temperature change found by Dearlove. T h e continuity of the line alters at 15, under which limit no regularity can be said to exist except approximately in the slope. T h e tests given here were obtained by bringing the cells up by intervals from o o , where they had remained over night, allowing ample time at each interval for
the attainment of a steady and uniform temperature. T h e divergence in the cells at oo is very large, amounting in the extreme to two millivolts, and persists for hours without showing any signs of change. T h e E. M. F. of the cells, however, converges when approaching 15', and from that point to 40" it shows perfect regularity. T h e return from 40' was also along the same line.
TABLE I11 Inversion of cadmium sulphate March 2 2 , 1897
15.0
14.2 12.0 10.1
8.2
0
1
1 1
5.2 2.0 2.0 O0
1
-0.17 $0.45 +0.69 0.97 71.34 1.62 $1.62
+ +
iI.89
In Table 111 some tests are given which were made to see whether a sudden change in E. M. F. could be noticed on coolThese results are ing a cadmium cell slowly from 15' to 0'. represented in Fig. 2 and show the effect on cells Cd5 and Cd,. Cells Cd4 and Cd, do not show any sudden change at all. T h e two former cells follow the slope of the temperature curve above 15' until about 2 O , when a sudden increase in E. 11. F. takes place. I n Fig. I on the ascending series (Cd4))a somewhat similar effect is seen to occur between 8' and 15'. T h e unstable E. 11.F. over this limited range, which is just about the mean temperature of the range of Jaeger and Wachsmuth's determinations, followks quite closely their temperature formula. It is evident that the point of actual crossing of the two branches of the curve must occur very nearly at 15' C. Various other tests were made to determine the temperature change above and
H. T. Barnes
346 t900 MV
I
Fig. I Temperature coefficient cadmium cells. J. & W. Formula proposed by Jaeger and TVachsmuth
+ 1-00
Fig. 2 Inversion of cadmium sulphate in the Weston cell
below 1 5 O , which were sufficient to satisfactorily verify the observations already given.
Of2
the Westoiz CeZZ
347
Ratio of Clark to Cadmium Not a little interest is attached to a determination of the ratio of the Clark to the Weston cell on account of the recent discussion of the absolute value of the Clark cell, which has such an important bearing on the absolute value of the mechanical equivalent of heat deduced by the electrical method of calorimetry. Dr. Kahle' has recently made some determinations of this ratio by means of the standard cells in the possession of the Reichsanstalt. T h e value of the ratio from a direct comparison of the Clark cell at I j" to the cadmium cell at 20" is given by him to be Clark 15' = 1.40663. Cad 20'
A comparison of cells Cd3, Cd4, Cd5 and Cd, with the B. 0. T. c i crystal" Clark cells described by the writer in another place2 was made by means of the cylinder potentiometer and the 6000-ohm galvanometer, which were used in some preliminary experiments on the comparison of the Clark cells.3 This potentiometer was repeatedly calibrated and its errors determined. Table IV contains the results of this comparison on the cells, which were all maintained at a constant temperature near 1 5 O C. TABLE IV March 6, 1897 Cell
x,
Cd,
corrected to
corrected for uniformity
6 8 2 2j
68290
48453
48575
Wied. Ann. 67, 35 (1899). 67, 151 (1897). Proc. Roy. SOC. loc. cit. p . IZI.
348
On the Weston Cell
cells and reducing the mean cadmium reading to 20' formula
E,= E,, - 0.086 ( t -
by the
IS),
the ratio of Clark to Weston is Clark 15' - 68294 - 1.40644, Cad 20' m 8 a value only slightly different to that given by Dr. Kahle. McGill Upziversity, April 6, 1900
I900 Since sending in the account of the measurements given above of the ratio of the Clark cell to the Weston, the author has had occasion to summarize all the data relative to the comparison of the original B. 0. T.''crystal" Clark cells, used in finding the ratio, which have been in the possession of the laboratory since 1895, with numerous other sets of cells made at different times since then. It has been found that the mean value of these original cells is systematically lower by 0.14 millivolt or I part in 10,000 than the means of all the other sets of cells, all of which agree very closely with each other. If full weight be given to these latter cells then the value of the ratio of Clark to Cadmium is increased to 1.40658. This value is so nearly identical with the value found for the ratio of the cells in the possession of the Reichsanstalt as to be, if not a coincidence, at least an indication of the" great accuracy attainable in the preparation of these standard cells by independent observers. N O T E ADDED MAY 22,
