' TO THE MOVEMENT OF MACHINES. 511 
direction of thé current is shown in fig. 3 by little arrows; it is re- 
versed each time the poles meet, provided the commutator be so 
placed that the edges of the levers shall quit one of the divisions in: 
order to pass to the other. This inversion acts, as is seen, instanta- 
neously, and quite independently of the velocity of rotation. The object 
is too simple, and sufficiently explained by the figures, to render it 
necessary to enter more into its details. I may add, further, that this 
same system of the inversion of the poles is applicable to any number 
of bars, provided that the sections of the discs are equal to them in’ 
number. I have constructed, for magneto-electric experiments, a 
double commutator of eight discs, with seventy-two divisions. In 
this apparatus there are also four levers similar to the former, which 
rest upon the cylinders (f') that unite the discs in pairs. The other- 
extremities of these levers are likewise immersed in jars of mercury, 
intended to receive the ends of a connecting wire, which is to be 
traversed by the voltaic or magneto-electric currents, sometimes in one. 
direction sometimes in the other. The instrument is put in motion by a 
handle, which can be easily turned twice in a second, and effect in the 
same time 144 double inversions. It will be easy to change or com- 
pletely interrupt the electric current 1000 or more times in a second. 
The nature of this current or of the magnetism wili of course be better 
understood by decomposing it into a rapid succession of pulsations: I 
am persuaded, for instance, that we should succeed by this means in 
charging a Leyden jar, or in effecting any chemical decompositions by 
the thermo-electric current of a single pair of elements. 
8. 
The magnetic power is produced and maintained, as is well known, by 
the action of the voltaic apparatus. By using zinc as a positive metal, 
copper as a negative metal, and water acidulated with sulphuric acid 
as the conducting liquid, it is the transformation of the metallic zine into 
sulphate of zinc which here constitutes the cost of keeping. the appa- 
ratus in action. -It is a matter of the greatest importance to reduce as’ 
much as possible this cost. Let us examine what is the relation be- 
tween the magnetism of the connecting wire and the action of the voltaic 
apparatus, Since the discovery of electro-magnetism this object has 
engaged the attention of distinguished scientific men, but it presents so. 
many difficulties and such a complication of circumstances, that we 
cannot be surprised that the theories and formulz which they have 
endeavoured to deduce from experiments differ considerably. 
. This is not the place to enter into the criticism of these theories ; but 
it appears to me that the theory established by M. Ohm, in a little work 
entitled “ Die galvanische Kette, mathematisch bearbeitet von Dr. G. S: 
Olan (1827),” and developed more fully in various memoirs printed 
in the German J ournals, presents so much simplicity, and agrees so well 
with all the phenomena of the voltaic pile, that I have not hesitated to 
