Why does voltage of pirmary circuit in potentiometer increase as we take more length of the potentiometer wire?
In this diagram it seems left end of the red wire (The blue dot) has highest potential and the PD should drop as we move to the right.

That would be correct if you took battery negative as your reference or &quot;ground&quot; point. In the illustration the author isn't measuring voltage from a ground point, s/he's just measuring difference between two points and the battery positive is the reference.



simulate this circuit &ndash; Schematic created using CircuitLab
Figure 1. (a) The author's circuit. (b) A more conventional circuit.
Imagine the resistors R1 and R2 as ladders with nine steps. The wiper is on the sixth step.

In (a) we're measuring the difference between the wiper and the top step so VM1 reads 9 - 6 = 3 V.
In (b) we're measuring the difference between ground and the wiper, 6 V.

The author is correct. The voltage reading will increase as the wiper / contact is moved to the right.