114 WRITINGS OP JOSEPH HENRY. [1838 



24. In all cases the several spires of the coil should be well 

 insulated, for although in magnetizing soft iron, and in anal- 

 ogous experiments, the touching of two spires is not attended 

 with any great reduction of action ; yet in the case of the 

 induced current, as will be shown in the progress of these 

 investigations, a single contact of two spires is sometimes 

 sufficient to neutralize the whole effect. 



25. It must be recollected that all the experiments with 

 these coils and helices, unless otherwise mentioned, are made 

 without the reaction of iron temporarily magnetized ; since 

 the introduction of this would in some cases interfere with 

 the action, and render the results more complex. 



SECTION II. 



Conditions which influence the production of Secondary Owrrents. 



26. The secondary currents, as it is well known, were dis- 

 covered in the introduction of magnetism and electricity, by 

 Dr. Faraday, in 1831. But he was at that time urged to the 

 exploration of new, and apparently richer veins of science, and 

 left this branch to be traced by others. Since then however 

 attention has been almost exclusively directed to one part of 

 the subject, namely, the induction from magnetism, and the 

 perfection of the magneto-electrical machine: and I know 

 of no attempts, except my own, to review and extend the 

 purely electrical part of Dr. Faraday's admirable discovery. 



27. The energetic action of the flat coil, in producing the 

 induction of a current on itself, led me to conclude that it 

 would also be the most proper means for the exhibition and 

 study of the phenomena of the secondary galvanic currents. 



28. For this purpose coil No. 1 was arranged to receive the 

 current from the small battery, and coil No. 2 placed on this, 

 with a plate of glass interposed to insure perfect insulation ; 

 as often as the circuit of No. 1 was interrupted, a powerful 

 secondary current was induced in No. 2. The arrangement 

 is the same as that exhibited in Fig. 3, with the exception 

 that in this the compound helix is represented as receiving 

 the induction, instead of coil No. 2. 



