510 PKOF. JACOBI ON THE APPLICATION OF ELECXKO-M AONETISM 



mercury is soon covered with a coating of oxide, which either entirely 

 ])revents the metallic contact, or at least weakens it. In employing 

 amalgamated surfaces this effect is produced still more rapidly. Be- 

 sides I have by incontestible proofs arrived at the conviction, that 

 the simple contact of metals with a clean surface is quite sufficient to 

 conduct the electric current, even of the weakest tension. The con- 

 tact by means of mercury adds nothing to the energy of this cur- 

 rent. It is erroneous to judge of this energy by the brilliancy of the 

 spark, proceeding only from the combustion of the mercury. I have 

 thought right to mention these circumstances, though apparently trifling. 

 In a motor, from which we look to obtain an infinitely accelerated mo- 

 tion, the smallest details should not be disregarded ; the most trivial are 

 ultimately of importance. 



7. 



Fig. 3. represents the Commutator, adapted to the magnetic apparatus, 

 so as to produce the inversion of the poles : a, h, c, d are four discs of 

 copper fixed upon the axis of rotation e e. The discs a, b and c, d are 

 united by copper tubes f, f, entirely insulated from the axis by the in- 

 terposition of a tube g, of varnished wood or any other insulating sub- 

 stance. 



The periphery of each disc is divided into eight exactly equal parts, 

 of which four h are cut into sectors and filled afterwards by pieces of 

 ebony, forming with the metal an accurate and smooth surface. The discs 

 are arranged upon the axis of rotation, so that the sectors of wood and 

 of metal alternately correspond, as represented by the shaded parts of the 

 figure. ZZ, CC are bars of copper, formed as levers, very move- 

 able in their supports : they are intended to conduct the current. 

 The arm of the longest lever forms at its extremity an edge, which rests 

 on the periphery of the corresponding disc. The other arm is bent and 

 plunged into a little jar filled with mercury, k. Tlie jars k k and k' k' 

 are united by plates of copper, as represented in figure 1 . The action of 

 this commutator will easily be understood. The levers are always in 

 contact with the discs, and are alternately so with the metallic and 

 insulating parts. By their mobility in their supports they yield to the 

 slightest inequality of the sui'face. and the friction they occasion is very 

 trifling. The helices which surround the moveable bars are united so 

 as to form a continuous wire, the ends of which l, m are soldered 

 respectively to the systems of the discs a, b and c, d. The other 

 helices, wound round the fixed bars, arc also united, and the ends n and 

 o immersed, the one in ajar of mercury jo, attached to the voltaic appa- 

 ratus, and the other in the jar k of the commutator. Thus all the six- 

 teen helices form only one connecting wire, through the medium of the 

 commutator. Tlie voltaic apparatus consists of four troughs of copper, 

 in which plates of zinc are immersed, all being united as in a pile. The 



