When exploring the relationship between the current on the resistance and the voltage at both ends, what should be kept unchanged?

When exploring the relationship between the current on the resistance and the voltage at both ends, what should be kept unchanged?

The resistance does not change

Explore the relationship between the current on the resistance and the voltage at both ends, urgent!
To explore the relationship between the current on the resistance and the voltage at both ends, why to use rheostat? What role does rheostat play in exploring the relationship between resistance and current, and between voltage and current

The use of rheostat 1. To prevent excessive current from damaging the resistance, that is, to protect the circuit 2. Partial voltage means that the rheostat bears part of the voltage, so that the resistance can reach the required voltage
Explore the resistance and current. Use two resistors in series. One is that the voltage at both ends of the rheostat does not change, which is to change the resistance value of the rheostat. Another one is that the voltage at both ends of the resistor will change, and the current change is obtained by I = u / R
Explore the voltage and current. In the general textbook experiment, the total voltage is constant. Change the resistance value of the rheostat, the voltage at both ends is constant, and change the current by I = u / R

When an electric iron works normally, its resistance is 72 ohm, and its current is 0.5A. Q: how large a resistance must be connected in series to connect it to 220 V
The calculation steps are given

Working voltage = 72 * 0.5 = 36V
The resistance needs to be divided into 220-36 = 184v
Resistance = 184 / 0.5 = 368 ohm