Wheatstone Bridge

Wheatstone Bridge

The device uses for the measurement of unknown values of the resistance with the help of comparison method is known as the Wheatstone bridge. The value of unknown resistance is easily determined by comparing it with the known resistance. Wheatstone bridge is used for the measurement of very low valued resistances.

 Using this special circuit, the value of any unknown resistance can be easily detected. The Wheatstone bridge works on principle of the  null deflection, i.e. in bridge the ratio of their resistances are equal, and no current flows through the galvanometer. In the Wheatstone bridge circuit  a diamond-shaped arrangement of four resistors. It has two parallel arms and each arm having two resistors in series. A third arm connected between the two parallel arms at some point within the arms.

Structure of Wheatstone bridge: shown in the Diagram.

It has four resistance according to the Diagram P, Q, R, S. Here the resistance P and Q are in the sequence and in the same way R and S are among the order categories. Then both the class combinations have been added in parallel with each other. The battery E is connected between the points a and c for electric carrier force. Among the four resistors, one resistance value can be easily determined by balancing the two arms. Out of four resistors, the value of two resistors P and R are known. The value of S is adjustable, and the value of Q is to be calculated.
Then this adjustment of resistance (Bridge) is connected to electric supply and a galvanometer between terminal b and terminal d.
Now the value of an adjustable resistor is adjusted (may increase or decrease) until the ratio of the two branches resistances become equal i.e. (P/ Q) = (R/S), and galvanometer reads zero  (zero current flowing through the circuit). Now the Bridge circuit is balanced and the value of the unknown resistor could be measured easily.


In the circuit, the battery of E functional carrier is connected, it is known that the main stream is emitted in the circuit, when it reaches the point point a, it gets two passes, so it divides into I1 & I2, then the I1 is at b point and I2 gets two routes again at d point, here three conditions are formed for the division of I1 & I2.

1. When the value of probability (Vb) at b point is greater than the probability (Vd) generated at d point i.e. Vb> Vd in this condition because the value of the probability is greater at b point and the probability of d at d point is less then b to d The stream will flow on the stream but the stream does not have to be of higher potential than the low potential, so the stream can not flow from d to b.  

So when Vb> Vd is in this position, I1 stream is divided on two sides, one part goes into the measurement measured and the second part Q resistance is less on the other side of the d point, hence the I2 stream does not go towards the stream marketer and The entire section goes into resistance                                                                                                                                                2. The second position will be the opposite of the first condition i.e.,at the point d,the probability can be greater and at the point B, i.e. Vb <Vd in this position, because the d is more likely to be d on the point, therefore the flow from the current from d to b but Can not move towards d.
Therefore, in this case, I2 stream is divided on two sides, one part goes into the measurement and the second part goes to the resistance and on the other side the point B is less and hence the I1 stream does not go towards the stream marketer and the whole Section Q goes into resistance.

3. In the third position, the value of probability at point b and d is same, i.e.  Vb = Vd in this condition since the potential is equal at both ends, so there is no stream on the side of the measurement which means there is no current flowing in bd because the current Different things need to be flowing and because there is a similar charge at both points here.
Therefore, in this situation, I1 is the full Q resistance on the current and the I2 stream reaches perfectly on the complete S resistance and there is no distraction due to the zero current in the stream meter, it is called the position of the balance of the bridge.

In the balance of the bridge
Vb = Vd
Va - Vb = Va - Vd
I1P = I2R
I1 / I2 = R / P

Other side
Vb = Vd

Vb - Vc = Vd - Vc
I1Q = I2S
I1 / I2 = S / Q
We compare the equations of I1 / I2
R / P = S / Q
This equation is used to find the value of an unknown resistance, as if assumed that the S resistance in the wheat bridge is unknown.
S = QR / P
Keeping the value of P, Q, R in it gets unknown S.
Thus, with the help of Wheatstone Bridge, the value of unknown resistance is known.

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