1.
Measurements done in a laboratory or at
some other place always involve errors.
 No measurement is free from error. If the
precision of the equipment is adequate, no
matter what its accuracy is, a discrepancy
will always be observed between two
measured results.

2.
 No measurement can be made with perfect
accuracy but it is important to find out what
accuracy actually is and how different errors
have entered into the measurement.
 A study of errors is a first step in finding ways
to reduce them. Errors may arise from
different sources and are usually classified as
under : ·
 1. Gross Errors.
 2. Systematic Errors.
 3. Random Errors,

3.
 This class of errors mainly covers human
mistakes in reading instruments and
recording and calculating measurement
results.
 The responsibility of the mistake normally lies
with the experimenter.
 The experimenter, may grossly misread the
scale.
 For example he may, due to an oversight,
read the temperature as 31.5°C while the
actual reading may be 21.5°C.

4.
 Gross errors may be of any amount and therefore
their mathematical analysis is impossible.
 However, they can be avoided by adopting two
means. They are :
1. Great care should be taken in reading and
recording the data.
2. Two, three or even more readings should be taken
for the quantity under measurements. These
readings should be taken preferably by different
experimenters and the readings should be taken at
a different reading point to avoid re-reading with
the same error.

5.
I. Instrumental Errors.
2. Environmental Errors.
3. Observational Errors.

6.
These errors arise due to three main
reasons :
(i) Due to inherent shortcomings in the
instrument
(ii) Due to misuse of the instruments,
(iii) Due to loading effects of instruments.

7.
 These errors are inherent in instruments
because of their mechanical structure.
 They may be due to construction, calibration
or operation of the instruments or measuring
devices.
 These errors may cause the instrument to
read too low or too high.
 For example, if the spring (used for producing
controlling torque) of a permanent magnet
instrument has become weak, the instrument
will always read high.

8.
The procedure of measurement must be
carefully planned. Substitution methods or
calibration against standards may be used
for the purpose.
Correction factors should be applied after
determining the instrumental errors.
The instrument may be re-calibrated
carefully.

9.
 There is an old saying that instruments are
better than the peop1e who use them.
 Too often, the errors caused in
measurements are due to the fault of the
operator than that of the instrument.
 A good instrument used in an unintelligent
way, may give erroneous results.
 Examples which may be cited for this misuse
of instrument may be failure to adjust the zero
of instruments, poor initial adjustments, using
leads of too high a resistance and so on.

10.
 One of the most common errors committed by
beginners, is the improper use of an instrument for
measurement work.
 For example, a well calibrated voltmeter may give
a misleading voltage reading when connected
across a high resistance circuit .
 The same voltmeter when connected in a low
resistance circuit, may give a more dependable
reading.
 These examples illustrate that the voltmeter has a
loading effect on the circuit, altering the actual
circuit conditions by the measurement process.

11.
These errors are due to conditions external
to the measuring device including
conditions in the area surrounding the
instrument.
These may be effects of temperature
pressure, humidity, dust, vibrations or of
external magnetic or electrostatic fields.

12.
 Arrangements are made to keep the
conditions as nearly as constant as possible.
For example, temperature can be kept
constant by keeping the equipment in a
temperature controlled enclosure.
 Using equipment which is immune to these
effects. For example, variations in resistance
with temperature can be minimized by using
resistance materials which have a very low
resistance temperature co-efficient.

13.
 Employing techniques which eliminate the
effects of these disturbances. For example,
the effect of humidity dust etc. can be entirely
eliminated by hermetically sealing the
equipment.
 Applying computed corrections: Efforts are
normally made to avoid the use of application
of computed corrections, but where these
corrections are needed and are necessary,
they are incorporated for the computations of
the results.

14.
There are many sources of observational
errors.
As an example, the pointer of a voltmeter
rests slightly above the surface of the
scale.
Thus an error on account of PARALLAX
will be incurred unless the line of vision of
the observer is exactly above the pointer.

15.
The quantity being measured is affected by
many happenings throughout the universe.
We are aware of and account for some of
the factors influencing the measurement
about the rest are unaware.
The happenings or disturbance about
which we are unaware are lumped and
called "Random" or "Residual”

16.
Moving coil type instruments
a) Permanent magnet type
b) Dynamometer type

17.
Principle :
when a current carrying conductor is placed in
magnetic field it is acted upon by a force which tends
to move it to one side and out of the field. This
movement of coil is used to measure current or
voltage.

18.
 This instrument consists of a
permanent magnet and a
rectangular coil of many turns
wound on a light aluminium or
copper former inside which is
an iron core
• The sides of the coil are free to
move in the two air gaps between
the poles and core
• To the moving coil spindle is
attached, a pointer is attached to the
spindle to move over a calibrated
scale.

19.
 A magnetic field of sufficient density is produced
by the permanent magnet.
 The moving coil carries the current or a current
proportional to the voltage to be measured.
 Hence, an electromagnetic force is produced
which tends to act on the moving coil and moves
it away from the field.
 This movement makes the spindle move and so
the pointer gives a proportionate deflection

20.
 Deflecting torque : It is directly proportional to the
current or the voltage to be measured. So, the
instrument can be used to measure direct current and dc
voltage.
 Control torque : Spring control.
 Damping torque : Eddy current damping.Damping is
electromagnetic by eddy currents induced in the metal
frame over which the coil is wound. Since the frame
moves in an intense magnetic field, the induced eddy
currents are large and damping is very effective.

21.
The permanent-magnet moving coil (PMMC) type instruments
have the following advantage and disadvantages:
ADVANTAGES
1. They have low power consumption
2. Their scales are uniform and can be designed to extend over and arc of 1700
degree or so
3. They possess high (torque/weight) ratio.
4. They can be modified what the help o f shunts and resistances to cover a wide
range of currents and voltages.
5. They have no hysteresis loss.
DISADVANTAGES
1. Due to delicate construction and the necessary accurate machining and
assembly of various parts, such instruments are somewhat costlier as
compared to moving iron instruments.
2. Some errors are set in due to the ageing of control springs and the permanent
magnets.

22.
Principle
An electrodynamic instrument is a moving coil instrument
in which the operating field is produced, not by a
permanent but by another fixed coil. This instrument can
be used either as an ammeter or voltmeter but is
generally used as a wattmeter.

23.
 Fixed coil (F) is made in two
sections.
 In the space between two,
Moving coil (M) is placed.
 Moving coil is attached to the
spindle to which pointer is
attached.
 The pointer is allowed to
move over a calibrated scale

24.
 The fixed coil and the moving coil carry
currents. Thus, two magnetic fields are
produced.
 Hence, an electromagnetic force tends to
act on the moving coil and makes it move.
This makes the pointer gives a
proportionate deflection.

25.
As voltmeter: The two coils are electrically in series. Deflecting torque
is proportional to square of voltage to be measured. Hence used
for measuring ac and dc voltages.
As ammeter: The two coils are electrically in series. Deflecting torque
is proportional to square of current to be measured. Hence used
for measuring ac and dc currents.
As wattmeter: Fixed coils carry the system current. Moving coil carries
a current proportional to the system voltage. The deflecting
torque is proportional to V ICos φ i.e. Power to be measured
Control torque : Spring control.
Damping torque : Air damping.