Measurement of voltmeter resistance using wheatstone’s bridge

AIM:

To  measure the resistance of the given dc voltmeter using Wheatstone’s Bridge and to find the multiplier resistor required to to extend the range of given voltmeter from (0-10)V to 100V.

EQUIPMENTS REQUIRED:

Wheatstone’s bridge Trainer Kit , Voltmeter, Connecting wires.

THEORY:

TheWheatstone’s bridge is widely used for precision measurement of resistance from approximately 1 ohm to the mega ohm range that is medium resistance. It operate upon the null deflection principle. The basic circuit of the wheatston’s  bridge is shown in the figure. It is the most accurate method available for measuring resistances and is popular for laboratory use.

The bridge has four resistive arms, together with a source of emf and null detector, usually a galvanometer. The current through the galvanometer depends on the potential difference between point C and D. When there is no current through the meter the galvanometer rest at zero. The basic circuit of a wheat stone bridge is shown in fig .it has four resistive arms, consisting of resistances P,Q,R and S together with a source of emf and a null detector , usually a galvanometer G or other sensitive current meter is used. 

PRECAUTIONS:

1. Connections should be tight.
2. Instrument should be handled carefully.

PROCEDURE:

1. Connect the patch cord as per the circuit diagram.
2. Voltmeter is connected across the unknown resistance R
3. Put the galvanometer and battery switches in internal position.
4. The galvanometer sensitivity is kept at the minimum position.
5. Fix the multiplier at constant value.
6. The knobs in the series arms are placed at minimum position.
7. The series arms knobs are varied until the galvanometer shows null deflection.

CIRCUIT DIAGRAM:

CALCULATIONS:

The voltmeter must be connected across the two points or a component, to measure the potential difference,  with the proper polarity. The multiplier resistance can be calculated as 

Rm = Internal resistance of coil .ie, meter

• V = Im (Rm+Rs)
• ImRs = V – ImRm

The multiplying factor for multiplier is the ratio of full range voltage to be measured and the drop across the basic meter.

:. m = Multiplying factor = V/v

       = Im (Rm+Rs)/ImRm

OBSERVATION:

RESULT :

Hence we have studied the low resistance by using whetstone bridge.

The Resistance of Given voltmeter measured using whetstone’s bridge is Rm = ………….ῼ