320 CONTRIBUTIONS TO ELECTRICITY AND MAGNETISM. 



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 bearing 

 on the improvement of the magneto-electrical machine: since the 

 plates of metal which sometimes forms the ends of the spool containing 

 the wire, must necessarily diminish the action, and also from experi- 

 ment of paragraph 72 the armature itself may circulate a closed cur- 

 rent which will interfere with the intensity 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 connec- 

 tion with several other points, in another communication to the So- 

 ciety. 



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 conductors interposed. 

 But from his method of performing the experiment, it is evident that 

 the plate of metal was placed between a straight conductor and the 

 needle. The arrangement was therefore 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 in contact, and the needle placed within 

 this, the effect would have been otherwise. 



SECTION y. 



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 current by 

 a secondary, in the interposed conductor, led me to conclude that if the 

 latter could be drawn out, or separated from the influence of the for- 

 mer, it would itself be capable of producing a new induced current in 

 a third conductor. 



