TO THE MOVEMENT OF MACHINES. 5 



be seen that there exists a distribution of magnetism in soUd 

 cylinders entirely distinct from that in hollow cylinders, so that 

 not only the mass, but the geometrical form also, must be 

 taken into account. 



28. 



The magneto-electric effects Avhich take place on completing 

 or breaking a voltaic circuit, are, as is well known, much more 

 decided when the connecting wire is bent into a helix, and still 

 more so, if this latter contains a soft iron nucleus. Whether 

 this nucleus was a solid or hollow tube, there was no perceptible 

 difference in the discharge or splendour of the spark of disjunc- 

 tion, even when the wire of the helix li line in thickness, or a 

 second coil of brass wire of 0*5 line diameter was employed. 

 These experiments were performed in the dark, in order to be 

 able to judge well of the splendour of the spark ; after each ex- 

 periment, the surface of the mercury, and the electrode [rcopliore] 

 inserted in it, were carefully cleaned. Moreover, I ascertained the 

 action of the pair, that constructed of zinc and silver, by the in- 

 terposition of a galvanometer which only indicated a variation 

 of from 35|^ to 35, and from 24^ to 24, occurring during the 

 course of the experiments. It is hardly necessary to add, that 

 the spark was scarcely perceptible as soon as the nucleus had 

 been removed. 



I made various combinations, Avhich it is unnecessary to describe 

 more in detail, to produce a magneto-electric current by the aid 

 of those tubes, sometimes by employing them as armatures of a 

 horse-shoe, sometimes as the nuclei of a magneto-electric helix. 

 The force of the current was indicated by the deviations of the 

 needle of an astatic galvanometer of Nobili. Although these ex- 

 periments were sufficiently delicate, considering that the least 

 accidents in the position of the ai-mature exert a certain influ- 

 ence on the deviation of the needle, yet I did not find any dif- 

 ference so notable as to be obliged to attribute it to some influ- 

 ence of the mass of the electro-motive cylinders. The law, 

 therefore, of Mr. Barlow may also be admitted with respect to 

 magneto-electric actions. But repeated experiments are neces- 

 sary to learn whether there are not conditions analogous to 

 those of Art. 27, and relative to a highly developed magnetic 

 state. I may be allowed to add that, had there been any no- 

 table difference in these experiments, it might justly have been 

 concluded that the magneto-electric current, or the extra-cun-ent 



