How to use potentiometer data from slave to master Arduino?
Start by reading the potentiometer. Add a Wire request service function and that makes the Slave:

#include &lt;Wire.h&gt;

const uint8_t controllerAddress = 8;
const int controllerPin = A0;
volatile int controllerValue = 0;

void setup() {
  Wire.begin(controllerAddress);
  Wire.onRequest(requestEvent); 
}

void loop() {
  int value = analogRead(controllerPin);
  noInterrupts(); 
    controllerValue = value;  
  interrupts();
}

void requestEvent() {
  Wire.write((const uint8_t*) &amp;controllerValue, sizeof(controllerValue)); 
}


This is minor update of the Arduino Wire Master Reader tutorial. The sketch for the Master is left as an exercise. 

Cheers!