26-02-2013, 10:49 AM
Measurement of Resistance using Wheatstone Bridge.
Measurement of Resistance.docx (Size: 13.16 KB / Downloads: 16)
APPARATUS:
•Wheatstone bridge
•Unknown resistance
•Connecting wires
THEORY:
A Wheatstone bridge is a measuring instrument used to measure an unknown electrical
resistance by balancing two legs of a bridge circuit, one leg of which includes the
unknown component.
In circuit diagram Rx is the unknown resistance to be measured; R1, R2 and R3 are resistors
of known resistance and the resistance of R2 is adjustable. If the ratio of the two
resistances in the known leg (R2 / R1) is equal to the ratio of the two in the unknown leg
(Rx / R3), then the voltage between the two midpoints (B and D) will be zero and no
current will flow through the galvanometer Vg. R2 is varied until this condition is reached.
The direction of the current indicates whether R2 is too high or too low.
Detecting zero current can be done to extremely high accuracy. Therefore, if R1, R2 and
R3 are known to high precision, then Rx can be measured to high precision. Very small
changes in Rx disrupt the balance and are readily detected.
At the point of balance, the ratio of R2 / R1 = Rx / R3
Therefore,
Alternatively, if R1, R2, and R3 are known, but R2 is not adjustable, the voltage difference
across or current flow through the meter can be used to calculate the value of Rx, using
Kirchhoff's circuit laws (also known as Kirchhoff's rules). This setup is frequently used in
strain gauge and resistance thermometer measurements, as it is usually faster to read a
voltage level off a meter than to adjust a resistance to zero the voltage.
PROCEDURE:
1. Connect the bridge terminal Rx to the unknown resistor.
2. Select a ratio R2/R1. If you know the range of Rx select R2/R1 near to a Rx for
the maximum sensitivity of the bridge. If you don’t know anything about Rx
select R1=1K and R2=1K.
3. Push the button M and check the movement of the pointer of the null detector.
4. If the pointer is out of the range put the sensitivity knob SENS on 1/K.
5. Rotate the knob R3 until the pointer of the galvanometer goes to zero.
6. Now increase the sensitivity of the galvanometer (SENS to 1/100, 1/10,1/1) and,
adjust the reference resistor R3, to obtain again the zero of the galvanometer.
7. With the maximum sensitivity and the galvanometer on the zero division we have
the following formula: