226 Contributions to Electricity and Magnetism. 



electrical discharge having destroyed the insulation at some point, 

 a part of the spires would thus form a shut circuit, and the induc- 

 tion in this would counteract the action in the other part of the 

 helix ; or, in other words, the helix was in the same condition as 

 the two helices mentioned in paragraph 70, when the ends of the 

 wire of one were joined. 



76. Also the same principle appears to have an important bear- 

 ing on the improvement of the magneto-electrical machine : 

 since the plates of metal which sometimes form the ends of the 

 spool containing the wire, must necessarily diminish the action, 

 and also from the experiment of paragraph 72, the armature itself 

 may circulate a closed current which will interfere with the in- 

 tensity of the induction in the surrounding wire. I am inclined 

 to believe that the increased effect observed by Sturgeon and 

 Calland, when a bundle of wire is substituted for a solid piece of 

 iron, is at least in part due to the interruption of these currents. 

 I hope to resume this part of the subject, in connection with seve- 

 ral other points, in another communication to the Society. 



77. The results given in this Section may, at first sight, be 

 thought at variance with the statements of Sir H. Davy, that 

 needles could be magnetized by an electrical discharge with con- 

 ductors interposed. But from his method of performing the ex- 

 periment, it is evident that the plate of metal was placed between 

 a straight conductor and the needle. The arrangement was there- 

 fore similar to the interrupted circuit in the experiment with the 

 cut plate (62,) which produces no screening effect. Had the 

 plate been curved into the form of a hollow cylinder, with the 

 two ends of the metal in contact, and the needle placed within 

 this, the effect would have been otherwise. 



SECTION v. 



On the Production and Properties of induced Currents of the 

 Third, Fourth and Fifth order. 



78. The fact of the perfect neutralization of the primary cur- 

 rent by a secondary, in the interposed conductor, led me to con- 

 clude that if the latter could be drawn out, or separated from the 

 influence of the former, it would itself be capable of producing a 

 new induced current in a third conductor. 



79. The arrangement exhibited in Fig. 8 furnishes a ready means 

 of testing this. The primary current, as usual, is passed through 



