c&i

1. Ex. No 1 (b)
Bridge Networks – AC Bridges
Aim
To measure the unknown value of low
inductance using bridge network with built in 1
kHz oscillator and headphone.
Apparatus Required
S.No. Apparatus Required Quantity
1. Maxwell bridge kit. 1 No
2. Audio Oscillator 1 No
3. Head phone 1 No
4. Unknown value inductance 1 No
5. Connecting wires As Required

2. • Circuit Diagram

3. Description
Maxwell’s Inductance Bridge
In this bridge an inductance is measured by comparison with a
standard variable capacitance are as,
Therefore, L = PRC ,P=1000,R=1000
P – Standard value of resistance in ohms
R – Standard value of resistance in ohms
C – Unknown value of capacitance in farads
х100

4. Tabulation
Model Calculation
S.No
.
Value of P (Ω)
Value of R
(Ω)
Value of
capacitance
(C)
Calculated
Value
Of L(H)
True value
(H) % Error
1.
1000 1000 0.151 151 150 -0.66%
C= (1*0.001)+(5*0.01)+(1*0.1)
= 0.151µF
L =PRC
= 1000*1000*0.151*10-6
=151mH
% ERROR = [(150-151) / 150 ] *100 = -0.66 %

5. Procedure
1.Connect the audio oscillator of 1 kHz of a variable output of about 0-
10 v to terminals are marked as oscillator.
2.A pair of sensitive headphone is connected to terminals marked
headphone.
3.An unknown inductance of about 50mH to 500 mH is connected to
terminals marked L.
4.Now switch on the oscillator and obtain for minimum sound in the
headphones by varying R&C.
5.It may not be possible to obtain complete silence due to self
capacitance of inductance coil and also due to presence of higher
harmonics.
6.So null point may be hired by getting only minimum sound in the
headphones.
7.Calculate the value of L by using the formula L=PRC.

6. Result
Thus the value of the given unknown inductance by using
Maxwell’s inductance bridge method was measured.

10. A Maxwell bridge is a modification to a Wheatstone bridge used to measure an
unknown inductance (usually of low Q value) in terms of calibrated resistance and
inductance or resistance and capacitance. When the calibrated components are a
parallel resistor and capacitor, the bridge is known as a Maxwell-Wien bridge
The advantages of a Maxwell Bridge are:
The frequency does not appear in the final expression of both equations, hence it
is independent of frequency.
Maxwell’s inductor capacitance bridge is very useful for the wide range of
measurement of inductor at audio frequencies
The disadvantages of a Maxwell Bridge are:
The variable standard capacitor is very expensive.
The bridge is limited to measurement of low quality coils (1 < Q < 10) and it is also
unsuitable for low value of Q (i.e. Q < 1) from this we conclude that a Maxwell bridge
is used suitable only for medium Q coils.