cant understand this circuit

nhrock3

the dot above the coil says that the voltage goes from top to buttom
i cant see how v3 needs to show is the sum or subraction of them both.

i can recognize the sum of two potentials when we connect the high potential of the first with the low potential of the second
and measure the other two ends with the volmeter
but he i cant see where they "connect the high potential of the first with the low potential of the second"
?
and where is the other ends?