ON ELECTRO-DYNAMIC INDUCTION. 335 



receive the induction, and its ends joined to those of the outer riband 

 of tinfoil of the glass cylinder, while the magnetizing spiral was at- 

 tached to the ends of the inner riband. A feeble tertiary current was 

 produced by this arrangement, which in two cases gave a polarity to 

 the needle indicating a direction the same as that of the primary cur- 

 rent. In other cases the magnetism was either imperceptible or 

 minus. With an arrangement of two coils of wires around two glass 

 cylinders, one within the other, the same effect v\'as produced. The 

 magnetism was less when the distance of the two sets of spires was 

 smaller, indicating, as it would appear, an approximation to a posi- 

 tion of neutrality. These results are rather of a negative kind, yet 

 they appear to indicate the same change with distance in the case of 

 the galvanic currents, as in that of the discharge of ordinary electricity. 

 The distance however at which the change takes place would seem to 

 be less in the former than in the latter. 



129. There is a perfect analogy between the inductive action of the 

 primary current from the galvanic apparatus and of that from the larger 

 electrical battery. The point of change, in each, appears to be at a 

 great distance. 



130. The neutralizing effect described in Section IV. may now be 

 more definitely explained by saying that when a third conductor is 

 acted on at the same time by a primary and secondary current (unless 

 it be very near the second wire) it will fall into the region of the plus 

 influence of the former, and into that of the minus influence of the 

 latter; and hence no induction will be produced. 



131. This will be rendered perfectly clear by Fig. 15, in which a 



p. jg represents the conductor of the primary 



+ a current, b that of the secondary, and c the 



+ i-q: ^ third conductor. The characters -j — f- 



' "^ Z ^ +5 &c., beginning at the middle of the 



+ — first conductor and extending downwards, 



■^ " represent the constant plus influence of 



the primary current, and those +0 , &c., beginning at the second 



conductor, indicate its inductive influence as changing with the distance. 

 The third conductor, as is shown by the figure, falls in the ^/ms region of 

 the primary current, and in the minus region of the secondary, and 

 Ti. — 4 I 



