984 
produces a deflection of the needle whose tangent is 
double or treble the first. 
The other method is by Dr Faraday’s Voltameter. 
Voltame- The amount of water decomposed is directly as the 
quantity of the current. The unit in this case is 
one cubic centimétre of gas produced from water in a 
minute, 
These two measures agree. After being once com- 
pared, we may in all cases deduce the decomposing 
force of a current from its effect upon a tangent com- 
ass. 
i Mr Wheatstone has facilitated the measurement 
of voltaic effects by the invention of the rheostat, a 
simple contrivance for introducing any desired length 
stoneand Of wire into a circuit, and thus estimating resistances 
both of conductors and of batteries, and also the 
electro-motive forces of different batteries. The re- 
sults appear to be extremely satisfactory. (Phil. 
Trans., 1843.) The conducting power - of different 
metals drawn into wire will be inversely as the 
lengths required to be introduced into the circuit to 
reduce the strength of the current in a given pro- 
portion. The principle of the rheostat was inde- 
pendently applied to similar inquiries by M. Jacobi 
of St Petersburg, in 1840. 
The laws of Ohm farther proceed to expound the 
effect of the size and number of the elementary cells 
combined in a voltaic battery. The size of the plates 
increases the quantity of electricity which escapes 
through a short conductor, but has little effect upon 
along current. On the other hand, the multiplica- 
tion of elements produces no increase in the voltaic 
stream when the connecting wire is short and when 
it is also a good conductor, for the chief resistance 
in the circuit is in that case the battery itself, which 
resistance increases with the number of elements, just 
as the force which overcomes it increases. If, on 
the other hand, the chief resistance be extraneous to 
the battery, the addition of more elements increases 
the power without much increasing the resistance. 
All this scarcely requires mathematical proof. It is 
very evident, and very just, and it is borne out by 
experiment, 
Ohm’s theory farther gives the partial effects of a 
current branching into various unequally good con- 
MATHEMATICAL AND PHYSICAL SCIENCE. 
(Diss. VI. 
ception in France and England is mainly due to the 
ingenious and (in many cases) independent experi- 
ments of M. Pouillet and Mr Wheatstone. 
Dante.u’s Constant Battery.—This seems the pro- vee 
per place to mention an invention which has exer-% 
cised a remarkable influence on the progress of prac- niell—the 
I constant 
believe that the merit of this application is entirely >#tery- 
tical electricity—I mean the Constant Battery. 
due to the late Professor Daniell,! although the Ger- 
man writers (who manifest throughout a singular 
sensibility with regard to their national claims to 
electrical improvements) seem to claim it for their 
countrymen. Every battery previously constructed 
diminished rapidly in energy from the instant of being 
charged, This was chiefly due to two causes; first, to 
the acid becoming gradually charged with oxide of 
zine; and, secondly, to the appearance of “ nascent” 
hydrogen arising from the decomposition of water at 
the copper surface where it prevented effectual con- 
duction of the electricity. These sources of di- 
minished effect were prevented in the following 
way :—Instead of a single cell containing one fluid 
moistening both the copper and the zinc, a double 
cell was formed by means of a partition of bladder 
or porous earthenware. The partition next the zine 
was filled with dilute sulphuric acid; the partition 
next the copper with a solution of sulphate of cop- 
per, also acidulated. When galvanic action proceeds, 
both fluids are decomposed ;. but whilst that in the 
zinc cell becomes charged with oxide of zine, it is 
at the same time continually acidulated by the 
electro-chemical transfer of acid from the decom- 
position of sulphate of copper in the copper cell; 
and the copper set free from the same combination 
in the form of oxide is metallically reduced by com- 
bining with the “nascent” hydrogen (the oxygen 
derived from the water decomposed having combined 
with the zinc), and the metallic copper is deposited in 
an ever fresh film on the surface of the copper plate. 
This beautiful invention was described in 1836. 
Many other batteries on the same principle have 
been since contrived and described; several are more 
powerful, but none perhaps are so constant in their 
action. 
ductors, and into other details, particularly as to the 
electric tension in different parts of a circuit. It is, 
however, to be observed, that the whole is’ based on 
the assumption that the dissipation of electricity from 
the surface of the conductor is insensible. 
(851.) It is only justice here to add, that the theory of 
Fechner’s Ohm owes much, if not most, of its value to the ex- 
&xperi- periments of Fechner in Germany; and that its re- 
Application of Electricity to the Arts—MM. 853.) 
Wueatstone and Jacost.—I have selected Messrs Applica- 
Charles Wheatstone and M. H. Jacobi as the TePTe> creatricity 
sentatives of a numerous class of ingenious men WhO to the arts. 
have shown great felicity of invention in applying in- 
genious mechanism to render electric agency available 
in the arts. We here again find the reciprocal influ- 
ments, ence of art upon science, to which I have elsewhere 
1 John Frederic Daniell, Professor of Chemistry in King’s College, London, was born in 1790. He was the author of a work 
on Chemical Philosophy, of Meteorological Essays, and of numerous papers in the Philosophical Transactions, many of which were | 
connected with Voltaic Action. His work on Meteorology contributed materially to the progress of that science, as did the in- 
vention of his Hygrometer (notwithstanding certain defects in that instrument) to the theory and practice of Hygrometry, For 
his Constant Battery Mr Daniell received the Copley medal of the Royal Society. His death tock place from apoplexy while 
attending a council meeting of the Royal Society on the 13th March 1845. 
