MATHEMATICAL AND PHYSICAL SCIENCE. 



[Diss. VI. 



produces a deflection of the needle whose tangent is 

 double or treble the first. 



(846.) The other method is by Dr Faraday's Voltameter. 



Voltame- The amount of water decomposed is directly as the 



ter " quantity of the current. The unit in this case is 



one cubic centimetre of gas produced from water in a 



minute. 



(847.) 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- 

 pass. 



(848.) Mr Wheatstone has facilitated the measurement 

 Rheostat O f voltaic effects by the invention of the rheostat, a 



f YT\T 



Wh t simple contrivance for introducing any desired length 

 stone and of wire into a circuit, and thus estimating resistances 

 Jacobi. 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. 



(849.) The laws of Ohm farther proceed to expound the 

 Farther effect of the size and number of the elementary cells 

 fromVhm's combined in a voltaic battery. The size of the plates 

 theory. increases the quantity of electricity which escapes 

 through a short conductor, but has little effect upon 

 a long 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. 



(850.) Ohm's theory farther gives the partial effects of a 

 current branching into various unequally good con- 

 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- 

 periments of Fechner in Germany ; and that its re- 



ception in France and England is mainly due to the 

 ingenious and (in many cases) independent experi- 

 ments of M. Pouillet and Mr Wheatstone. 



DANIELL'S Constant Battery. This seems the pro- (852.) 

 per place to mention an invention which has exer- ^ p" 

 cised a remarkable influence on the progress of prac- n i e ii_the 

 tical electricity I mean the Constant Battery. I constant 

 believe that the merit of this application is entirely batterv - 

 due to the late Professor Daniell, 1 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 

 zinc ; 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 partjjtion of bladder 

 or porous earthenware. The partition next the zinc 

 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 zinc, 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. 



Application of Electricity to the Arts MM. (853.) 

 WHEATSTONE and JACOBI. I have selected Messrs Applica- 

 Charles Wheatstone and M. H. Jacobi as the wpre- JJjJjjJ,^ 

 sentatives of a numerous class of ingenious men who to t h e 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- 

 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 took place from apoplexy while 

 attending a council meeting of the Royal Society on the 13th March 1845. 



