(1) When studying the relationship between multiple physical quantities, the control variable method is adopted. In order to verify the relationship between its resistance and the length of the new material, the voltage should be kept constant and the length of the new material connected to the circuit should be changed.

(2) According to the data in the table, the greater the length of the conductor, the greater the resistance value, so the conclusion is that the resistance of the conductor made of this material is related to its length.

So the answer is: (1) voltage; The length of new material connected to the circuit; (2) The resistance of a conductor made of this material is related to its length.