TO THE MOVEMENT OF MACHIXES. 511 



direction of the eurrcnt is shown in fig. 3 by little arrows ; it is re- 

 versed each time the poles meet, provided the commutator be so 

 placed Ihat 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 (y) 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 will 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 zinc into 

 9ul])hate 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 compliciition of circumstances, that we 

 cannot be surprised that the theories and formulae which they have 

 endeavoured to deduce from experiments differ considerably. 



.This is not the place to enter into the crit'eism of these theories ; but 

 it appears to me that the theory established by M. Ohm, in a little work 

 entitled " Die galvanisclui Kette, mathemutisch bearbeitet von Dr. G. S: 

 Ohm (1827)," and developed more fully in various memoirs printed 

 in the German Journals, presents so nmch sini)>licity, and agrees so well 

 with all the i)l)a;iioinena of the voltaic i)ile, that I have not liesitated to 



