508 Prof. J. Henry's Contributions 



constant quantity, and is not affected by changes in these con- 

 ditions; hence, if we suppose the induction which produces 

 the secondary current to be sufficiently intense, the velocity of 

 its development will exceed that of its diminution, as in the 

 example of the primary current from the intense source of 

 the compound battery of many elements. Now this is the 

 case with the inductions which produce currents of the dif- 

 ferent orders, capable of giving shocks or of magnetizing steel 

 needles : the secondary currents from these are always of con- 

 siderable intensity, and hence their rate of development must 

 be o-i-eater than that of their diminution; and, consequently, 

 they may be represented by a curve of the form exhibited in 

 fitr. 20, in which there is no constant part, and in which the 

 ° steepness of A B is greater 



than that of B C. There 

 are, however, other con- 



siderations, which will be 

 noticed hereafter (89.), 

 which may affect the form 

 of the part B C 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 develop- 

 ment in one direction, it consists of two instantaneous cur- 

 rents, one produced by the induction of A B, and the other, 

 by that of B C, in opposite directions, of equal quantities, but 

 of different intensities. The whole quantity of induction in 

 the two directions will each be represented by the ordinate 

 B Z», and hence they will nearly neutralize each other, in re- 

 ference to their action on the galvanometer, 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 A B ; 

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

 turned in the other direction, by the action of B C. This in- 

 ference is in strict accordance with observation : the needle, 

 as we have seen (24.), starts from a state of rest, with a velo- 

 city which, apiJarenlly, 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 



