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 diminution, 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 
Fig. 20. 
ep 
sefetkn  F: 
A ob C 
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 Bb, 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. he 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, a parently, would send it through a large arc, but before 
it has re shed, 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. 
Se : 
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