Fan. 19, 1882] 
NATURE 
279 
me by Mr. Groves of Bolsover Street, the above equation 
becomes (using the circular measure for the sine) 
We OAR ON / 77 at of tax ee oy (2) 
in which V is the difference of potential between the poles of the 
battery, and # is the number of turns of the micrometer screw 
from the vertical position (in which @=0) to the limiting position 
of equilibrium, or that in which the disk is ast sestained out of 
contact with the screws. 
The disk is observed from behind the guard plate by means of 
a microscope attached rigidly to the plate and moving, of course, 
with it. The slightest motion of the disk can be thus seen; 
and when, by tilting the plates, the attraction N ceases to cause 
a motion of the disk, we know that the limiting inclination @ is 
attained. 
As there would be great difficulty in determining the vertical 
po-ition of the plates accurately, I do not seek to determine it, 
but use a differential method. Thus, suppose that we use 
cells in each of which the E.M.F. is E, and let the reading of 
the micrometer be # when the limiting position is reached ; now 
use 7’ of the cells, and let the reading for equilibrium be 7’ ; 
then we know m—m’ but not # and m’ separately, Substituting 
ni for V in equation (2), we have 
n*E? = (0456)? 
Also 
2% = ("0456)?7’. 
w. E=0456,/_4, . . @) 
Le 
where A stands for 7-7’. 
Of course it is obvious that, to reduce any error of reading to 
a minimum, it is advisable to use for x and » a large number 
and a small number, respectively. With the present instrument 
it is not possible to use more than about fifty Leclanché cells, 
because these produce such a large displacement of the disk that 
the amount of play allowed is exhausted. 
A series of experiments carried out last summer in Prof. G. 
Carey Foster’s laboratory gave for the E.M.¥. of each cell of a 
battery of fifty Leclanchés ‘00475 absolute electrostatic units. 
This was obtained on the supposition that the E.M.F. was 
the same in all the cells, a supposition which is extremely 
improbable. 
Within the last few days I have repeated the experiments on a 
different principle. My first idea was to work with a battery of 
Grove elements. Each element consisted of a test-tube containing 
a saturated solution of sulphate of zinc, and inside this a smaller 
test tube containing nitric acid, both test-tubes having the same 
axis, and both fitted into a paraffin cork, or stopper. A little 
zinc rod (surrounded with a very thin glass tube open at its lower 
end) plunged into the liquid of the outer test-tube and came up 
through the paraffin stopper ; a platinum wire came up similarly 
out of the nitric acid; and electrical communication between 
the liquids was maintained by an aperture in the inner test-tube, 
through which the fumes of the nitric acid passed into the 
outer, 
The resistance of the cells was, of course, enormous. They 
were formed into a battery, and supported in a wooden board 
soaked with paraffin, 
The result then obtained for the E.M.F. of the Grove was 
much below what I knew to be about its value. The reason of 
this appeared to be that, with the great internal resistance of the 
battery, the external resistance was not sufficiently great. I | 
therefore diminished the internal, and at the same time increased 
the external, resistance by inserting threads of asbestos through 
the apertures in the inner test-tubes, the extremities of every 
thread dipping into both liquids, and also by suspending each 
cell separately by a fine silk thread, about 2 feet long, froma 
fixed horizontal glass rod. The result was an increased, but still 
unsatisfactory, value of the E,M.F., and the unsatisfactory result 
was due to the fact that the nitric acid gradually attacked the 
zine rods. 
The employment of cells of exceedingly high resistance for the 
measurement of electromotive force is open to the serious objec- 
tion that with them it is necessary to have a practically infinite 
external resistance, and this it isnot always easy to attain. Even 
with the Thomson quadrant electrometer such cells give an un- 
certain re-ult. When we have to trust for conduction to fumes 
or a moist film between two glass vessels containing the liquids, 
we occasionally get no indication whatever from the electrometer, 
and it is only by shaking up the cells that the requisite 
conductivity is obtained. 
The above form of battery was abandoned for a series of 
chloride of zinc elements. Here the internal resistance is com- 
paratively small, but we must not assume all the cells to have 
the same E.M.F. I therefore took forty of these elements, and 
compared their electromotive forces by a Thomson quadrant 
electrometer. In this way I found a variation of more than § 
per cent. in the E.M.F. of two cells. 
Denote the electromotive forces of the cells by E,, E,, Es, 
. . .. and let D stand for the electromotive force of a given 
Daniell, or any other element whose E.M.F. is to be found abso- 
lutely ; and let the vatios of E,, E,, . . . . to D, as determined 
by a Thomson quadrant electrometer, be 74, 7), 73...» 
Now suppose that we use any number of the cells with the 
absolute sine electrometer, and that the sum of their electromotive 
forces, (7, + 7%, +73 +.. .)D, is denoted by D.37. Note the 
reading of the micrometer screw when the limiting deflection of 
the plates is reached. Then use a smaller number, whose total 
E.M.F. is represented by D.o7, and take the new reading. If 
A is the difference of readings, we have by equation (3) 
ee Mae 
\D= 60) ee Reyes) 
The ratios 7, 7), . . » must, of course, be marked on pieces of 
paper attached to the outsides of the corresponding cells. 
I quote the result of the measurement of the E.M.F. of a par- 
ticular Daniell. Taking observations with 39 of the chloride of 
zine cells and with 10 of them I found 
A=14°2; 
also the registered values of the ratios 74, 7%, . » . gave in this 
case 
Sr=50'427D ; ov=12'834D ; 
and by substituting in equation (4) we have 
IDO RI Gg Gun fo 0 (©) 
absolute electrostatic units as the E.M.F. of the Daniell. 
Again, taking observations with 39 and 20 of the cells, I 
found 
Rarer 7 
Sr=50°427D ; or=25'918, 
and these numbers substituted in (4) give exactly the value (a) 
above. 
It is not possible, with the present instrument, to work with 
two batches of cells differing slightly in number ; for I find that 
in some cases I cannot be certain of the reading corre-ponding 
to limiting equilibrium within about one-fifth of a revolution of 
the screw head. This uncertainty is of no consequence when A 
is large ; but it is capable, I believe, of being almost completely 
got rid of. 
Sir William Thomson’s final estimate of the E.M.F. of a 
Daniell is 
*00374 
absolute electrostatic units (‘‘ Electrostatics and Magnetism,” 
p. 246). : ‘ 
The Daniell cell used in the above experiments was a parti- 
cular form of ‘‘gravity” arrangement, and I have good reason 
to believe that its E.M.F. was somewhat below that of a normal 
Daniell. Hence the value obiained for its E,.M.F, may be quite 
consistent with Sir William Thomson’s number. : 
I hope before long to determine by means of the absolute sine 
electrometer the E.M.F. of a cell which is also known in electro- 
