helix in which a magnet is inserted. This is the explanation of the problem of the unifonn rapidity of the magnetic machine; for, being put in motion by the magnetizing power of a voltaic current, it represents simultaneously an apparatus composed of magnets in motion, and capable of producing a magneto-electric current, in a direction contrary to the voltaic current. The first is closed by the pile itself, which, being composed of a single pair only, does not offer too great a resistance to its passage.
In the connecting wire, formed by the union of the sixteen helices of the apparatus, I interposed a galvanometer; and then, by closing the circuit and preventing the motion of the machine, I observed the deviation of the needle: it amounted to nearly 60°. As soon as the motion of the apparatus commenced the needle began to recede, and continued to do so more and more as the speed became more accelerated. The motion having become uniform, at the rate of 60 revolutions in a minute, the needle became stationary at a deviation of about 47°. The needle always advanced when the motion was stopped or retarded; it receded, on the contrary, when it was mechanically accelerated. It appears that the deviation of the needle of 47° corresponds with the state of equilibrium; for the motion having of itself ceased, the needle did not quit this position. Thus in the different experiments, whether the first deviation of the needle exceeded 60°, or was less, it always became fixed at about 47°. The voltaic current having been weakened by the interposition of different branches, until the first deviation amounted only to 47°, the magnetism was not sufficiently strong to produce the movement of the apparatus. Repeated experiments will be necessary to investigate these interesting phænomena.
19.
I imagined that it would be useful to open two passages or two separate branches to the magneto-electric current; one of which should be the pile, and the other a second connecting metallic wire, so long and so thin as not too much to affect the quantity of electricity passing through the principal connecting wire. (Art. 8., No. 6.) There was reason to suppose that the counter-current would rather follow the metallic wire than the liquid of the pile: but it was not so. During the motion of the apparatus, the needle of the galvanometer being fixed at 47°, and the second circuit having been suddenly established, the needle was not much affected by it. It advanced, it is true, but only 1°·5. Neither did the speed of the apparatus sensibly change. On reducing the length of the second wire it was nearly the same. The passage of the counter-current across the metallic wire was proved, at least in part, by the interposition of a second galvanometer. During the accelerated movement the needle of this latter ad-
