180 Dr. Faraday's Experimental Researches in Electricity. 



734-. In. many cases when the instrument is used as a com- 

 parative standard, or even as a measurer, it may be desirable 

 to collect the hydrogen only, as being less liable to absorption 

 or disappearance in other ways than the oxygen ; whilst at the 

 same time its volume is so large, as to render it a good and 

 sensible indicator. In such cases the first and second form 

 of apparatus have been used, figg. 7, 8. (707. 708.). The in- 

 dications obtained were very constant, the variations being 

 much smaller than in those forms of apparatus collecting both 

 gases; and they can also be procured when solutions are used 

 in comparative experiments, which, yielding no oxygen or 

 only secondary results of its action, can give no indications if 

 the educts at both electrodes be collected. Such is the case 

 when solutions of ammonia, muriatic acid, chlorides, iodides, 

 acetates, or other vegetable salts, &c, are employed. 



735. In a few cases, as where solutions of metallic salts 

 liable to reduction at the negative electrode are acted upon, 

 the oxygen may be advantageously used as the measuring 

 substance. This is the case, for instance, with sulphate of 

 copper. 



736. There are therefore two general forms of the instru- 

 ment which I submit as a measurer of electricity. One, in 

 which both the gases of the water decomposed are collected 

 (709. 710. 711.) ; and the other, in which a single gas, as the 

 hydrogen only, is used (707. 708.). When referred to as a 

 comparative instrument, (a use I shall now make of it very ex- 

 tensively,) it will not often require particular precaution in 

 the observation ; but when used as an absolute measurer, it will 

 be needful that the barometric pressure and the temperature 

 be taken into account, and that the graduation of the instru- 

 ments should be to one scale; the hundredths and smaller di- 

 visions of a cubical inch are quite fit for this purpose, and the 

 hundredth may be very conveniently taken as indicating a 

 degree of electricity. 



737. It can scarcely be needful to point out further than has 

 been done how this instrument is to be used. It is to be in- 

 troduced into the course of the electric current, the action of 

 which is to be exerted anywhere else, and if 60° or 70° of 

 electricity are to be measured out, either in one or several 

 portions, the current, whether strong or weak, is to be con- 

 tinued until the gas in the tube occupies that number of divi- 

 sions or hundredths of a cubical inch. Or if a quantity com- 

 petent to produce a certain effect is to be measured, the effect 

 is to be obtained, and then the indication read off. In exact 

 experiments it is necessary to correct the volume of gas for 



