This circuit is not wrong. However, there is one problem with this design: At certain positions of the potentiometer the sensor will become saturated while at other positions the transistor will become saturated. You can change the transistor to a MOSFET. The same problem will occur. As you rotate the potentiometer two possibilities can occur:
1. The sensor saturates before the transistor.
2. The transistor saturates before the sensor.
Once either sensor and transistor is saturated you cannot detect the change in current.
