Measurement Technique
Report on
Kelvin Double Bridge for
Low resistance
measurement
AIM OF EXPERIMENT:To study the Kelvin Double Bridge for Low resistance measurement.
OBJECTIVE:Kelvin's double bridge may be used for precision measurement of four-terminal low
resistances. Four terminal resistors have two current leading terminals and two potential
terminals across which the resistance equals the marked nominal value. This is because the
current must enter and leave the resistor in a fashion that there is the same or equivalent
distribution of current density between the particular equipotent surfaces used to define
the resistance. The additional points also eliminated any contact resistance at the current
lead-in terminals.
EQUIPMENT REQUIRED:-
SL NO.
NAME
SPECIFICATION
QUANTITY
AL-E326B
1
1
KELVIN’S DOUBLE BRIDGE TRAINER
KIT
2
DIGITAL MULTIMETER
1
3
CONNECTING WIRES
AS REQ.
THEORY:Kelvin bridge (also called a Kelvin double bridge and in some countries a Thomson bridge)
is a measuring instrument used to measure unknown electrical resistors below 1 ohm. It is
specifically designed to measure resistors that are constructed as four terminal resistors.
Resistors above about 1 ohm in value can be measured using a variety of techniques, such
as an ohmmeter or by using a Wheatstone Bridge. In such resistors, the resistance of the
connecting wires or terminals is negligible compared to the resistance value. For resistors
of less than an ohm, the resistance of the connecting wires or terminals becomes
significant, and conventional measurement techniques will include them in the result.
To overcome the problems of these undesirable resistances (known as 'parasitic
resistance"), very low value resistors and particularly precision resistors and high current
ammeter shunts are constructed as four terminal resistors. These resistances have a pair
of current terminals and a pair of potential or voltage terminals. In use, a current is passed
between the current terminals, but the volt drop across the resistor is measured at the
potential terminals. The volt drop measured will be entirely due to the resistor itself as the
parasitic resistance of the leads carrying the current to and from the resistor are not
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included in the potential circuit. To measure such resistances, requires a bridge circuit
designed
to work with four terminal resistances. That bridge is the Kelvin bridge.
Since Galvanometer is a very sensitive instrument, therefore it can not measure high
potential difference. In order to convert a Galvanometer into voltmeter, a very high
resistance known as "series resistance" is connected in series with the galvanometer. Let
resistance of galvanometer = R, and resistance R, (high) is connected in series to it. Then
combined resistance = (R+ Rx).
PROCEDURE:1. THe trainer kit is switched on.
2. Connection was made as per connection diagram.
3. Variable DC power supply to the circuit i.e (0-15V) was given.
4. Any unknown value between (0.1-1e) was selected.
5. Then with the help of variable resistance i.e R1 and R3 was balanced.
6. When digital galvanometer shows voltage near (0.00-2.00 mv). Then the bridge was
balanced.
OBSERVATION:-
SL
NO.
R1 in Ω
1
631K
100
2
898K
3
4
3
R2 in Ω
R3 in Ω
RX(theoretical) RX(practical)
% error
in Ω
in Ω
603
0.1
0.096
4
250
741
0.2
0.206
3
877K
250
958
0.3
0.273
8.96
652K
330
897
0.4
0.45
13.5
Schematic diagram of kelvin double bridge trainer kit.
4
CALCULATION:Sl no.:- 1
R1= 631K Ω
R2= 100Ω
R3=603Ω
RX(theoretical)= 0.1Ω
RX(practical)= (603*100)/(631*103)=0.096Ω
5
% error= 4%
Sl no.:- 2
R1= 898K Ω
R2= 250Ω
R3=741Ω
RX(theoretical)= 0.2Ω
RX(practical)= (741*250)/(898*103)=0.206Ω
% error= 3%
Sl no.:- 3
R1= 877K Ω
R2= 250Ω
R3=958Ω
RX(theoretical)= 0.3Ω
RX(practical)= (958*250)/(877*103)=0.273Ω
% error= 8.98%
Sl no.:- 4
R1= 652K Ω
R2= 330Ω
R3=897Ω
RX(theoretical)= 0.4Ω
RX(practical)= (897*330)/(652*103)=0.45Ω
6
% error= 13.5%
CONCLUSION:We successfully calculated the value of small resistance. We also found the percentage
error between theoretical value and practical value. We also conclude that the sensitivity of
Kelvin’s double bridge is 0.1µΩ to 1Ω.
Group members:1. Sambit Sahu
2. Amit kumar Swain
3. Pratyush priyabrata Mishra
4. Dipjyoti Mishra
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