2 JACOBI ON THE APPI-ICATION OF ELECTRO-MAGNETISM 



If Ave first consider the experiments enumerated by M. Par- 

 rot, p. 123 of the Bulletin, it appears that ^ve may deduce 

 from them the important result, that the sohd thickness of 

 hollow tubes has but little influence on their magnetic power. 

 In fact, the magnetism of the tube designated by B, of 21 lines 

 solid thickness, differs merely -gV in magnetic power from the 

 tube D, which is but one line in thickness. Even the energy of 

 the solid cylinder of equal surface is far below the proi)ortion re- 

 lative to its mass. These experiments are connected with the 

 beautiful known law established by Mr. Barlow, and which re- 

 lates to the action of teiTCstrial magnetism on solid and hollow 

 soft iron spheres, a law which is also confirmed by the experi- 

 ments of Captain Kater on sheet-iron tubes, and which has un- 

 dergone the rigorous and scientific calculations of M. Poisson. 

 Mr. Barlow fixed the limit of the solid thickness at about Jj^ of 

 an inch ; such is the cylinder F of M. Parrot, constructed of 

 common sheet-iron, of -^j of an inch in thickness, and the result 

 of which differs considerably from other experiments. Its mag- 

 netic power amounted only to 32A, while that of the tube D of 

 one line in solid thickness is expressed by 89. 



26. 



We ought to observe extreme caution in drawing conclusions 

 from the experiments in question. The manner of comparing 

 magnetic forces from the adherence of the armature is itself sub- 

 ject to serious inconveniences, which consist, for the most part, 

 in the complicated accidents which accompany the contact of the 

 armature and its sudden interruption. The errors which proceed 

 from this cause being very considerable, it is at least worth while 

 to endeavour to separate them from others depending on the 

 continual variation of the voltaic action, a source not less proli- 

 fic of sei'ious errors. The somewhat difficult experiments insti- 

 tuted on the magnetic force of a soft iron bar, bent in a horse- 

 shoe form, and which I have related in Art. 9. of this me- 

 moir, made me s jnsible of these inconveniences in all their ex- 

 tent. But they may easily be overcome, by keeping an exact 

 account of the force of the current, by means of a galvanometer 

 interposed in the circuit. I had constructed, purposely for such 

 experiments, an instrument of very great service. A needle. 



