In the Test class you have the following loop at the end:
for (int ohm = 0; ohm <=500; ohm+=100) {
c5 = new Potentiometer(ohm);
Circuit cd = new Parallel(c12345, d);
System.out.println(c5.getOhm());
System.out.println(cd.getOhm());
}
I believe your question is: why is it that, even though I am changing the resistance in the potentiometer c5 for every iteration of the loop, that the resistance for the circuit cd is not changing.
If I am interpreting that right, then the answer is that circuit cd consists of circuits c12345 and d. And since neither c12345 or d is variable, neither will the resistance of cd vary.
I think you meant to make cd out of c12345 and Potentiometer *c5*. If you do that, then the output resistance will vary with each iteration.