To obtain the balance condition in a Wheatstone bridge circuit, the resistance arms R1 and R2 are varied.

Explanation:
A Wheatstone bridge circuit is a type of electrical circuit used to measure an unknown resistance by comparing it with known resistances. The circuit consists of four resistors connected in a diamond shape, with an unknown resistance (R4) in one arm and a standard known resistance (R3) in the opposite arm.

The balance condition in a Wheatstone bridge circuit occurs when the potential difference between the two midpoints of the bridge (junctions A and B) is zero. This condition can be determined by varying the resistance arms R1 and R2.

Here is how the balance condition is obtained by varying the resistance arms:

1. Initially, the Wheatstone bridge circuit is set up with known values of resistors R1, R2, and R3, and an unknown resistance R4.

2. A voltage source is connected across the bridge, and a galvanometer (null detector) is connected between junctions A and B.

3. The resistors R1 and R2 are varied until the galvanometer shows zero deflection or null. This indicates that the potential difference between junctions A and B is zero, which is the condition for balance.

4. When the balance condition is achieved, the ratio of the resistances in the bridge can be used to calculate the value of the unknown resistance R4.

5. By varying the resistance arms R1 and R2, the balance condition can be obtained for any value of the unknown resistance R4. This allows for accurate measurement of the unknown resistance.

In summary, the balance condition in a Wheatstone bridge circuit is obtained by varying the resistance arms R1 and R2 until the galvanometer shows zero deflection. This ensures that the potential difference between junctions A and B is zero, indicating a balanced condition.