How do you verify that resistance of a conductor is proportional to the length of the conductor for constant cross-section area and temperature?

A circuit is prepared by using a cell, key, ammeter as shown in below figure.

Iron wires of different lengths but same cross-sectional area were taken. Connect one wire between terminals P and Q. The ammeter reading was noted. The same procedure was repeated for other wires. We will observe that current decreases with increase in the length of wire. Resistance increases with increase in length and current decreases. Thus we can prove resistance of a conductor is proportional to the length of the conductor for constant cross-section area and temperature.