86 DR. FARADAY'S EXPERIMENTAL RESEARCHES IN ELECTRICITY. 



passed, or in other circumstances. The conchisive proofs of the truth of these state- 

 ments shall be given almost immediately (783. &c.). 



705. I endeavoured upon this law to construct an instrument which should measure 

 out the electricity passing through it, and which, being interposed in the course of the 

 current used in any particular experiment, should serve at pleasure, either as a com- 

 parative standard of effect, or as a positive measurer of this subtile agent. 



706. There is no substance better fitted, under ordinary circumstances, to be the 

 indicating body in such an instrument than water ; for it is decomposed with facility 

 when rendered a better conductor by the addition of acids or salts ; its elements may 

 in numerous cases be obtained and collected without any embarrassment from secon 

 dary action, and, being gaseous, they are in the best physical condition for separation 

 and measurement. Water, therefore, acidulated by sulphuric acid, is the substance 

 I shall generally refer to, although it may become expedient in peculiar cases or forms 

 of experiment to use other bodies (843.). 



707. The first precaution needful in the construction of the instrument was to 

 avoid the recombination of the evolved gases, an effect which the positive electrode 

 has been found so capable of producing (571.). For this purpose various forms of 

 decomposing apparatus were used. The first consisted of straight tubes, each con- 

 taining a plate and wire of platina soldered together by gold, and fixed hermetically 

 in the glass at the closed extremity of the tube (Plate I. fig. 5.). The tubes were 

 about eight inches long, 0*7 of an inch in diameter, and graduated. The platina plates 

 were about an inch long, as wide as the tubes would permit, and adjusted as near to 

 the mouths of the tubes as was consistent with the safe collection of the gases evolved. 

 In certain cases, where it was required to evolve the elements upon as small a surface 

 as possible, the metallic extremity, instead of being a plate, consisted of the wire bent 

 into the form of a ring (fig. 6.). When these tubes were used as measurers, they were 

 filled with the dilute sulphuric acid, and inverted in a basin of the same liquid (fig. 7.), 

 being placed in an inclined position, with their mouths near to each other, that as 

 little decomposing matter should intervene as possible ; and also, in such a direction 

 that the platina plates should be in vertical planes (720.). 



708. Another form of apparatus was that delineated (fig. 8.). The tube is bent in 

 the middle ; one end is closed ; in that end is fixed a wire and plate, a, proceeding so 

 far downwards, that, when in the position figured, it shall be as near to the angle as 

 possible, consistently with the collection, at the closed extremity of the tube, of all the 

 gas evolved against it. The plane of this plate is also perpendicular (720.). The 

 other metallic termination, 5, is introduced at the time decomposition is to be effected, 

 being brought as near the angle as possible, without causing any gas to pass from it 

 towards the closed end of the instrument. The gas evolved against it is allowed to 

 escape. 



709. The third form of apparatus contains both electrodes in the same tube ; the 

 transmission, therefore, of the electricity, and the consequent decomposition, is far 



