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CHAPTER 2
Direct –Current Indicating Instruments
1 Suspension Galvanometer
It is a device used to detect and measure small current in the circuit.
Principle: -A current carrying coil placed in a magnetic feld
Experiences a torque
Construction: - It consists o a narrow rectangular coil having a large number
o turns o copper wound on a non-metallic rame. It is suspended between the
semicircular poles pieces o powerul horseshoe magnet by mean o a
suspension wire made o phosphor bronze with a needle or pointer attached toit. A cylindrical sot iron core is fxed within the coil. his ma!es the magnetic
feld lin!ed with coil to be radial. At the lower end" coil is attached to a fne
spring. #amp and and $cale arrangement is used to measure de%ection
&ig'
2 Torue and De!lection o! t"e Galvanometer
(hen the current I is passed through the coil" it experiences a torque
which is
given as
T
d
(
d
e
f
e
c
ti
n
g
t
o
r
q
u
e
)=
N
I
A
B
(
h
e
r
e
)
*
n
o
o
t
ur
n
s
A *
area
coil
+ * magnetic feld" and
,ue to this torque" coil starts rotating. $o the suspension wire gets twisted and a restoring
torque is developed in the suspension wire made o hosphor bronze. he restoring torquecontinues to increase and stage reaches when restoring torque becomes equal to the
de%ecting torque.
At equilibrium Defecting Torque = Restoring Torque
or controlling torque
Td=Tc
N I A B = k θ
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(here θ is the angle by which the coil is rotated and ! is the restoring torque per unit
angle .
hereore I = k θ NBA = ! θ
(here ! = k NBA is called galvanometer constant.
I ∝ θ
ence current is directly proportional to the angle by which the coil rotated. Also light
rom lamp alls on the mirror gets de%ected and alls on the scale. (hen current passes
the mirror moves and light spot moves on the scale.
# Permanent $%agnet moving –coil mec"anism &P%%C'
In //0 meter or 1,2Arsonval3 meter or galvanometer all are the sameinstrument" a coil o fne wire is suspended in a magnetic feld producedby permanent magnet. According to the&undamental law o electromagnetic orce" the coil will rotate in themagnetic feld when it
0arries an electric current by electromagnetic 1E/3 torque e4ect. A pointerwhich attached the /ovable coil will de%ect according to the amount o current to be measured which applied to the coil. he 1E/3 torque iscounterbalance by the mechanical torque o control springs attached to themovable coil also. (hen the torques are balanced the moving coil willstopped and its angular de%ection represent the amount o electrical currentto be measured against a fxed reerence" called a scale. I the permanentmagnet feld is uniorm and the spring linear" then the pointer de%ection isalso linear.
( Galvanometer Sensitivit)
"ensiti#it$' - A galvanometer is said to be sensitive" i it give largede%ection" even when a small current is passed through it or a small voltageis applied across its coil.Current "ensiti#it$:-It is defned as the de%ection produced in the
galvanometer on passing a current o one ampere. i.e.0urrent $ensitivity *%olt&ge "ensiti#it$: - It is defned as the de%ection produced in thegalvanometer when a potential di4erence o one volt is applied its coil.
*oltage Sensitivit) +
, DC Ammeters
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Indirect D.C Ammeter:
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Have to determine values o! Ra-R. and R. to measure di!!erent range o!
currents
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/ DC *oltmeters
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0 *oltmeter Sensitivit)
8 Voltmeter- Ammeter method
This is very popular method for measurement of medium resistances since instrumentsrequired for this method are usually available in laboratory. The two types of connections
employed for ammeter voltmeter method are shown in figure. In both the methods if
readings of ammeter and voltmeter are taken then we can measure value of resistance by
using formula:
The measured value of resistance Rm, would be equal to the true value R, if the ammeter
resistance is zero and the voltmeter resistance is infinite, so that the conditions in the circuit are
not disturbed. ut in actual practice this is not possible and hence both methods give inaccurateresults.
Consider circuit of figure (a):
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In this method ammeter measures the true value of current flowing through resistance but
voltmeter does not measures the true value of the voltage across the resistance. the voltmeter
indicates the sum of the voltage across resistance and ammeter.!et Ra be the resistance of the ammeter
It is clear from the above equation that the error will be small if the value of the measuring
resistance is large as compare to the internal resistance of the ammeter .therefore circuit should
be used when measuring resistances are high.
Consider circuit of figure (b):
voltmeter ammeter method
In this circuit the voltmeter measures the true value of the voltage across the measuring
resistance but the ammeter does not measures the true value of the current flowing through theresistance .the current through the ammeter is the sum of the current through the voltmeter and
resistance.
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!et Rv be the resistance of the voltmeter.
rom t"e a.ove euation it is clear t"e true value o! t"e resistance ill .e eual to t"e measured
value onl) "en t"e voltmeter resistance is eual to t"e in!inite$ Hoever- i! t"e resistance o! t"e
voltmeter is ver) large as compared to t"e resistance under measurement3
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4 Series –t)pe 5"mmeter
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16 S"unt –t)pe 5"mmeter
11 %illimeter or *5% Cali.ration o! Dc Instruments3t"is is a instrument is used to measure
current -voltage and Resistance$