What is the difference between the ammeter connection of two circuit diagrams in the volt ampere characteristic test of nonlinear resistance?

What is the difference between the ammeter connection of two circuit diagrams in the volt ampere characteristic test of nonlinear resistance?

(1) Small internal resistance (2) volt ampere characteristic curve is close to the linear DC power supply (3) an inclined straight line through the origin. My physics teacher should be careful!

The current in the circuit is about 0.4A and the resistance to be measured is about 10 Ω

6V power supply, 0.6A ammeter, 6V voltmeter, 10 ohm sliding rheostat;
Current limiting type, ammeter external connection method

Experimental circuit diagram of resistance measurement by voltammetry

Does the current have anything to do with the resistance?
I've always been obsessed with this. The size of the resistance is only related to the material, length, cross-sectional area and temperature. When the voltage and current change, the size of the resistance will not change, because it is its own nature, but sometimes it doesn't seem to be so. I hope you can tell me the connection between them in some cases, so that I can understand them,

For example: I = 6a, u = 12V, r = 2 Ω, R: I = 1:3u = 2V, I = 1a, r = 2 Ω, R: I = 2:1r = 1 Ω, I = 5V, R: I = 1:5r = 10 Ω, I = 5a, u = 50V, R: I