Refer Figure 13.22(a) from the textbook.
Consider the magnetic flux is decreasing and it is flowing in downward direction.
Consider the voltage at terminal is and voltage at terminal is.
According to Lenz’s law, the current would flow in the loop in such a way that it would oppose the flow of flux inside it. Thus, the current should flow from point b to point a.
Calculate the voltage difference.
is the voltage difference between and .
Therefore, the polarity of.