1. The Kelvin bridge is a modified Wheatstone bridge used to accurately measure low resistances. It accounts for contact resistance by using four-terminal connections. 2. In a Kelvin bridge, the unknown resistance is connected between two potential terminals to directly measure its value, independent of contact resistance at the current terminals. 3. The bridge balances when the galvanometer is connected to a point between the potential terminals that divides the lead resistance into two equal parts, nullifying its effect on the measurement.

1. Measurement of Resistance

2. Classification of Resistances
According to
Nature of Supply
DC Resistance
Ohm’s law : I α V
Where, V= p.d. across the resistance
I= current flowing through that resistor
AC Resistance
Skin Effect: Cross section area of wire
reduced, this implies effective R
increases
Proximity Effect: Several conductors
are together, this implies effective
resistance is higher than the that if
conductors were alone
According to
Value of
Resistances
Low Resistance (R < 1 Ω) (Resistance of conductor)
e.g. Resistance series field winding of a D.C. series
generator, resistance of armature winding of generator,
earth wire resistance
Medium Resistance (1Ω < R < 0.1 MΩ)
e.g. Resistance of field winding of D.C. shunt generator,
resistance of long transmission line
High Resistance (R > 0.1 MΩ) (Resistance of insulator)
e.g. Resistance of cable insulation, resistance of
insulator disk of transmission line

3. Measurement of High Resistance
• Direct Deflection Method
• Loss of charge method
• Ohmmeter
• Megger

4. Loss of Charge Method
In loss of charge method unknown
resistance is connected in parallel
with capacitor and electrostatic
voltmeter.
The capacitor is initially charged to
some suitable voltage by means of a
battery of voltage V and then allowed
to discharge through the resistance.
The terminal voltage is observed
during discharge and it is given by,
V=v exp (-t/CR)
=>V/v = exp (-t/CR)
=>R= t/(C ln (V/v))
= 0.4343 t / (C log (V/v))

5. Continued….
 The variation of voltage v with time is
shown in figure, loss of charge
method plot. From above equation it
follows that if V, v, C and t are known
the value of R can be computed.
 If the resistance R is very large the
time for an appreciable fall in voltage
is very large and thus this process
may become time consuming.
 Also the voltage-time curve will thus
be very flat and unless great care is
taken in measuring voltages at the
beginning and at the end of time t, a
serious error may be made in the
ratio V/v causing the considerable
corresponding error in the measured
value of R.

6. Continued……
 more accurate results may be obtained by change in the voltage V-v
directly and calling this change as e, the expression for R becomes:
 This change in voltage may be measured by a galvanometer.
However, from the experimental point of view, it may be advisable
to determine the time t from the discharge curve of the capacitor
by plotting curve of log v against time t. this curve is linear as
shown in second figure and thus determination of time t from this
curve for the voltage to fall from V to v yields more accurate
results.
 Loss of charge method is applicable to some high resistances, but it
requires a capacitor of very high leakage resistance as high as
resistance being measured..

7. Ohmmeters
• Direct reading of the device for measurement
of resistance
• Low degree of accuracy
• Useful for find out the approximate resistance
of circuit components such as heater
elements, checking semiconductor diodes,
checking continuity of circuits.
• Series type ohmmeter
• Shunt type ohmmeter

8. Series Type Ohmmeters
• It consists of basic d’Arsonval
movement meter connected
in parallel with a shunting
resistor R2. This parallel circuit
is in series with resistance R1
and a battery of emf E.
• The series circuit is connected
to the terminals A and B of
unknown resistor Rx.
• R1 = current limiting resistor,
R2 = zero adjusting resistor,
E = emf of internal battery,
Rm = internal resistance of
d’Arsonval movement

9. Continued…..
• When the unknown resistance Rx = 0 (terminals A and B shorted) maximum
current flows through the meter. Under this condition resistor R2 is
adjusted until the basic movement meter indicates full scale current Ifsd. The
full scale current position of the pointer is marked “0Ω” on the scale.
• Similarly when Rx is removed from circuit Rx = ∞ (that is when
terminal A and B are open), the current in the meter drops to the
zero and the movement indicates zero current which is the marked
“∞”.
• Thus the meter will read infinite resistance at the zero current
position and zero resistance at full scale current position. Since zero
resistance is indicated when current in the meter is the maximum
and hence the pointer goes to the top mark.
• When the unknown resistance is inserted at terminal A, B the current
through the meter is reduced and hence pointer drops lower on the
scale. Therefore the meter has “0” at extreme right and “∞” at the
extreme left.
• Intermediate scale marking may be placed on the scale by different
known values of the resistance Rx to the instrument.

10. Megger

11. Construction of Megger
• Deflecting & Control coil : Connected parallel to the generator,
mounted at right angle to each other and maintain polarities in
such a way to produced torque in opposite direction.
• Permanent Magnets: Produce magnetic field to deflect pointer
with North-South pole magnet.
• Pointer : One end of the pointer connected with coil another end
deflects on scale from infinity to zero.
• Scale : A scale is provided in front-top of the megger from range
‘zero’ to ‘infinity’, enable us to read the value.
• D.C generator or Battery connection : Testing voltage is produced
by hand operated D.C generator for manual operated Megger.
Battery / electronic voltage charger is provided for automatic type
Megger for same purpose.
• Pressure coil resistance and Current coil resistance : Protect
instrument from any damage because of low external electrical
resistance under test.

12. Working of Megger
• Voltage for testing produced by hand operated megger by rotation
of crank in case of hand operated type, a battery is used for
electronic tester.
• 500 Volt DC is sufficient for performing test on equipment range up
to 440 Volts.
• 1000V to 5000V is used for testing for high voltage electrical
systems.
• Deflecting coil or current coil connected in series and allows
flowing the electric current taken by the circuit being tested.
• The control coil also known as pressure coil is connected across the
circuit.
• Current limiting resistor (CCR & PCR ) connected in series with
control & deflecting coil to protect damage in case of very low
resistance in external circuit.

13. Continued…..
• In hand operated megger electromagnetic induction effect is used to
produce the test voltage i.e. armature arranges to move in permanent
magnetic field or vice versa. Where as in electronic type megger battery
are used to produce the testing voltage.
• As the voltage increases in external circuit the deflection of pointer
increases and deflection of pointer decreases with a increases of
current.
• Hence, resultant torque is directly proportional to voltage & inversely
proportional to current.
• When electrical circuit being tested is open, torque due to voltage coil
will be maximum & pointer shows ‘infinity’ means no shorting
throughout the circuit and has maximum resistance within the circuit
under test.
• If there is short circuit pointer shows ‘zero’, which means ‘NO’
resistance within circuit being tested.

14. Continued…..
• Work philosophy based on ohm-meter. The deflection torque is
produced with megger tester due to the magnetic field produced by
voltage & current, similarly like ‘Ohm‘s Law’.
• Torque of the megger varies in ration with V/I, (Ohm's Law :- V=IR
or R=V/I). Electrical resistance to be measured is connected across
the generator & in series with deflecting coil. Produced torque shall
be in opposite direction if current supplied to the coil.
• High resistance = No current:- No current shall flow through
deflecting coil, if resistance is very high i.e. infinity position of
pointer.
• Small resistance = High current:- If circuit measures small
resistance allows a high electric current to pass through deflecting
coil, i.e. produced torque make the pointer to set at ‘ZERO’.
• Intermediate resistance = varied current:- If measured resistance is
intermediate, produced torque align or set the pointer between
the range of ‘ZERO to INIFINITY’.

15. Measurements of Low Resistance
• Kelvin Bridge
• Kelvin’s Double Bridge Method
• Voltmeter and Ammeter Method

16. Construction of Low Resistances
• Low resistance is the
resistance whose value is
less than 1 ohm.
• When these resistances are
connected in the circuit
used for measurements,
contact resistance affect
the measured value of low
resistance.
• To avoid this, low
resistances are constructed
with four terminals.

17. Continued…..
• Terminal marked as CC’ are known as current terminals and are
used to carry current to and from the resistance.
• The other two terminals marked as PP’ are known as potential
terminals and voltage drop across the resistance is measured
between these two terminals.
• The value of resistance is measured between the potential
terminals P and P’.
• This value is more accurate as it is independent of the contact
resistance, at the CC’ current terminals.

18. Kelvin Bridge
• Kelvin Bridge is a modified
Wheatstone bridge and
provides high accuracy
especially in the measurement
of low resistance.
• It is the portion of leads and
contacts where we must do
modification, because of these
there is an increment in net
resistance.
• Where,
C = the unknown resistance.
D = the standard resistance
(whose value is known).

19. • Let us mark the two points j and k. If the galvanometer is connected
to j point the resistance t is added to D which results in too low
value of C.
• Now we connect galvanometer to k point it would result in high
value of unknown resistance C.
• Let us connect the galvanometer to point d which is lying in
between j and k such that d divides t into ratio t1 and t2, now from
the above figure it can be seen that
Continued…..

20. Continued…..
• Thus we can conclude that there is no effect of t (i.e. resistance of
leads). Practically it is impossible to have such situation however
the above simple modification suggests that the galvanometer can
be connected between these points j and k so as to obtain the null
point.