when making a resistance vs voltage chart of variable resistors connected to different kinds of circuits. Measuring the resistance of the variable pots wiper position compared to the output voltage of the circuit has a different curve compared to the linear resistance of the pots wiper. The variable resistor is linear but the output voltage of the circuit is nonlinear voltage or has a voltage curve. The resistance rotation that is linear is not proportional with the output voltage of a circuit when plotting the instantaneous voltages against the variable pots rotation resistance. Any reasons why circuits change a linear pot into a nonlinear or is not proportional with the output voltage of a circuit or network. I'm not sure what type of test this is called or done but I'm sure R&D engineers do it when prototyping boards.