126 WRITINGS OF JOSEPH HENRY. [1838 



75. Before proceeding to the next Section, I may here 

 state that the phenomenon mentioned, paragraph 54, in ref- 

 erence to helix No. 4, is connected with the neutralizing ac- 

 tion. The electrical discharge having destroyed the insula- 

 tion at some point, a part of the spires would thus form a 

 shut circuit, and the induction 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 

 bearing on the improvement of the magneto-electrical ma- 

 chine : since the plates of metal which sometimes form the 

 ends of the spool containing the wire, must necessarily di- 

 minish the action, and also from experiment of paragraph 72 

 the armature itself may circulate a closed current which 

 will interfere with the intensity of the induction in the sur- 

 rounding wire. I am inclined to believe that the increased 

 ejBfect 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 several 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 conductors interposed. But from his method of per- 

 forming 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. 



