182 Dr. Tyndall on the Progress of the Physical Sciences i 



action to that of the one leg of the same U upon the other, and 

 prevent its extension from U to U, each alternate U was bent 

 upward, so as to form an angle of 50 or 60 degrees with the 

 horizon. A large induction-disc was placed in the primary cir- 

 cuit, and a corresponding one in the secondary circuit, which, 

 together with this, contained the thermometer and one of the 

 lengths of copper wire just described. The current through the 

 stretched -out wire was first proved, then through the spiral, and 

 finally through the system of Us. The following are the results 

 obtained : — 



The strength of the secondary current, when no action takes 

 place between its various parts, is thus shown to be interme- 

 diate between those of the N-form and U-form. In passing 

 through the former, the current is weakened in the ratio of 

 100 : 60, while in passing through the latter it is strengthened 

 in the ratio of 100 : 114. The law of the primary current is 

 therefore applicable to the secondary :- — Two portions of a 

 secondary current which run closely parallel, act upon each other i 

 when the directions of the current through both portions are iden- 

 tical, a weakening is the consequence ; and when the directions in 

 both portions are opposed to each other, a strengthening of the cur- 

 rent is the result. 



What is the cause of this ? During his investigation of the 

 primary current, M. Riess conjectured that the action of the pri- 

 mary upon itself was due to the formation of a secondary cui'rent 

 in the primaiy wire. Consistent with this view, we should infer 

 that the action of the secondary upon itself is due to the forma- 

 tion of a tertiary current in the mass of the secondary wire, an 

 inference which the author has established experimentally in the 

 following manner : — It has already been shown that a secondary 

 current is greatly weakened if the portion of the primary wire 



