VARIOUS FORMS OF THE VOLTA-ELECTROMETER. 87 



more rapid than in the separate tubes. The resulting gas is the sum of the portions 

 evolved at the two electrodes, and the instrument is better adapted than either of the 

 former as a measurer of the quantity of voltaic electricity transmitted in ordinary cases. 

 It consists of a straight tube (fig. 9.) closed at the upper extremity, and graduated, 

 through the sides of which pass the platina wires (being fused into the glass), which 

 are connected with two plates within. The tube is fitted by grinding into one mouth 

 of a double-necked bottle. If the latter be one half or two thirds full of the dilute sul- 

 phuric acid, it will, upon inclination of the whole, flow into the tube and fill it. When 

 an electric current is passed through the instrument, the gases evolved against the 

 plates collect in the upper portion of the tube, and are not subject to the recombining 

 power of the platina. 



710. Another form of the instrument is given at fig. 10. 



711. A fifth form is delineated (fig. 11.). This I have found exceedingly useful in 

 experiments continued in succession for days together, and where large quantities of 

 indicating gas were to be collected. It is fixed on a weighted foot, and has the form 

 of a small retort containing the two electrodes : the neck is narrow, and sufficiently 

 long to deliver gas issuing from it into a jar placed in a small pneumatic trough. The 

 electrode chamber, sealed hermetically at the part held in the stand, is five inches in 

 length, and 06 of an inch in diameter ; the neck about nine inches in length, and 

 0-4 of an inch in diameter internally. The figure will fully indicate the construction. 



712. It can hardly be requisite to remark, that in the arrangement of any of these 

 forms of apparatus, they, and the wires connecting them with the substance, which is 

 collaterally subjected to the action of the same electric current, should be so far in- 

 sulated as to ensure a certainty that all the electricity which passes through the one 

 shall also be transmitted through the other. 



713. Next to the precaution of collecting the gases, if mingled, out of contact with 

 the platinum, was the necessity of testing the law of a definite electrolytic action, 

 upon water at least, under all varieties of condition ; that, with a conviction of its 

 certainty, might also be obtained a knowledge of those interfering circumstances 

 which would require to be practically guarded against. 



714. The first point investigated was the influence or indifference of extensive 

 variations in the size of the electrodes, for which purpose instruments like those last 

 described (709. 710. 711.) were used. One of these had plates 0*7 of an inch wide, and 

 nearly four inches long ; another had plates only 0*5 of an inch wide, and 0*8 of an 

 inch long ; a third had wires 0*02 of an inch in diameter, and three inches long ; and 

 a fourth similar wires only half an inch in length. Yet when these were filled with 

 dilute sulphuric acid, and, being placed in succession, had one common current of 

 electricity passed through them, very nearly the same quantity of gas was evolved in 

 all. The difference was sometimes in favour of one, and sometimes on the side of 

 another ; but the general result was that the largest quantity of gases was evolved 

 upon the smaller surface of the wires. 



