Measurement of ammeter resistance using kelvin’s double bridge

AIM:

To determine the resistance of the given coil of ammeter using Kelvin’s Double bridge and to extend the range of ammeter from (0-5)A to 100 A.

EQUIPMENTS REQUIRED:

Kelvin’sdouble bridge Trainer Kit , Ammeter, Galvanometer , Connecting wires.

THEORY:

The Kelvin’s Double Bridge is a modification of the wheat stone Bridge (WB) and provides increased accuracy in the measurement of low resistance’s. The resistance’s of the lead and contact resistance of which is a major source of error in the Wheatstone Bridge  is overcome in this method. 

The Kelvin’s Double Bridge incorporates the idea of a second set of ratio arms – hence the name Double Bridge – and the use of four – terminal resistor for the law resistance arms. As shown in the figure the first of ratio arms is P & Q. The second set of ratio arms, P and V, is used to connect the galvanometer to point D at the appropriate potential between points M and N to eliminate the effect of connecting lead of resistance R between the known resistance R and the Standard resistance S. 

PRECAUTIONS:

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

PROCEDURE:

1. The ammeter is connected across +P and –P using a thick wire of very short length. 
2. +P , +C as well as –P ,-C is short circuited.
3. The current is adjusted by varying R to a value which is less than the maximum value of the ammeter.
4. The ammeter resistance is measured using kelvin’s bridge.
5. The resistance of wire can also be measured by replacing the ammeter by a wire .

CIRCUIT DIAGRAM:

CALCULATIONS:

The shunt resistance can be calculated as :

• Rm = Internal resistance of coil
• Rsh = Shunt resistance
• Im = full scale deflection current
• Ish = Shunt current
• I = Total current = I = Ish + Im

As the two resistance are in parallel voltage drop across them are equal;

:. IshRsh = ImRm

:. Rsh =ImRm/Ish

Where m = I/Im

Here M is called multiplying power of the shunt and defined as the ratio of total current to the current through the coil. It can be expressed as ,

m = 1 + (Rm/Rsh)

The shunt resistance may consist of a constant temperature resistance wire within the case of the meter or it may be external shunt having low resistance.

Resistance of the coil ,Rm = Range * [ (Reading of dial 1) +( Reading of dial 2 X 0.001)]

OBSERVATION:

Sl .No.	Multiplier setting	Reading	Unknown Resistance

RESULT:

The resistance of the coil of the wire is measured using Kelvin’s Double Bridge and the value of Resistance ,Rm = …………ῼ