260 Prof. J. Henry's Contributions 



tion. The electrical discharge having destroyed the insulation 

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

 circuit, and the induction in this would counteract the ac- 

 tion 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 expei'iment 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 

 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 connexion 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 pro- 

 duces 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 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 

 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 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 coil No. 1, while coil No. 2 is placed over this 

 to receive the induction with its ends joined to those of coil 

 No. 3. By this disposition the secondary current passes 

 through No. 3 ; and since this is at a distance, and without 



