146 Contributions to Electricity and Magnetism. 



ders, capable of giving shocks or of magnetizing steel needles; 

 the secondary currents from these are always of considerable in- 

 tensity, and hence their rate of development must be greater 

 than that of their diminntion, and, consequently, they may be 

 represented by a curve of the form exhibited in Fig. 20, in which 

 there is no constant part, and in which the steepness of AB is 



Fiji. 20. 



A 



greater than that of BC. There are, however, other considera- 

 tions, which will be noticed hereafter, (89,) which may affect the 

 form of the part BC of the curve. Fig. 20, rendering it still more 

 gradual in its descent, or, in other words, which tend to diminish 

 the intensity of the ending induction of the secondary current. 



79. It will be seen at once, by an inspection of the curve, that 

 the effect produced, in a third conductor, and which we have 

 called a tertiary current, is not of the same nature as that of a 

 secondary current. Instead of being a single development in one 

 direction, it consists of two instantaneous currents, one produced 

 by the induction of AB, and the other, by that of BC, in oppo- 

 site directions, of equal quantities, but of different intensities. 

 The whole quantity of induction in the two directions, will each 

 be represented by the ordinate B6, and hence they will nearly 

 neutralize each other, in reference to their action on the galva- 

 nometer, in the circuit of the third conductor. I say, they will 

 nearly neutralize each other, because, although they are equal 

 in quantity, they do not both act in absolutely the same moment 

 of time. The needle will, therefore, be slightly affected ; it will 

 be impelled in one direction, say to the right, by the induction of 

 AB, but, before it can get fairly under way, it will be arrested, 

 and turned in the other direction, by the action of BC. This 

 inference is in strict accordance with observation ; the needle, as 

 we have seen, (24,) starts from a state of rest, with a velocity 

 which, apparently, would send it through a large arc, but before 

 it has reached, perhaps, more than half a degree, it suddenly 

 stops, and turns in the other direction. As the needle is first 

 affected by the action of AB, it indicates a current in the adverse 

 direction to the secondary current. . 



