TO THE MOVEMENT OF MACHINES. 525 
have reckoned, in another series of experiments, the time which elapsed 
in developing 10 bubbles of air. The following is the table: 
Derigton othe agement of builes of | POVUAHETEN eager af 0 bubs 
34° 30! Q9""5 18° 29! so! 
32° 30! 95! 15° 15! 101" 
31° O75 14° 30! 124! 
99° 56" 14° 20! 126" 
21° 37! a } 14° JO! 129" 
g1° 29! 59" | 13° 90! 147! 
19° 37! 67" |° 13° 160" 
It is necessary to remark, that there was also a very feeble develop- 
ment of gas even at the surface of the zinc, which was taken into 
account. But the quantity of gas measured was, I believe, less than 
the quantity of gas developed; for there was a secondary action, which 
was manifested by the blackness of the plate of silver, and which we 
must attribute to a metallic reduction of the oxides dissolved in the 
acid. As it is very difficult to translate into forces the deviation of 
the needle*, these tables will not tend to confirm the law of Mr. Fa- 
raday: they only show that the deviation of the needle follows the 
same course as the development of the gas. I shall repeat the expe- 
riments, but reversing the process; that is to say, the development of 
gas will be taken for the most exact measure of the force of the 
current, and the value of the degrees of the galvanometer will be 
deduced from it, either immediately, or by some formula of interpo- 
lation of convenient application. ‘The experiments cited are not suf- 
ficiently rigorous to form the elements of calculation. 
17. 
To return to the magnetic machine. We had succeeded in obtaining 
an inversior of the direction of the current, both instantaneous and ex- 
act, by the commutator described above in article7, the effect of which is 
not at all affected by the quickness of rotation. We had even succeeded 
in obtaining, at least for some time, a tolerably constant voltaic apparatus. 
In short, the means have been discovered of reducing the expense of 
maintenance to a minimum, by preventing the direct action of the acid 
upon the zine, an action which cannot be turned to any use, and which, 
as is known, greatly surpasses that which serves to produce the 
voltaic current. Thus the most important difficulties in the practical 
application of electro-magnetism being overcome, it appeared to me 
time to examine more closely the nature of the forces which I desired 
to put in use, and principally to seek for the cause which limits a 
* Becquerel’s Treatise on Electricity and Magnetism, vol. ii. p. 20. 
