Page 815 - Basic Electrical Engineering

In null-type instruments, a null or zero indication of the pointer is used as
criteria for determining the value of an unknown quantity. Generally null-
type instruments are uncalibrated instruments with the pointer placed on the

middle of the graduated but uncalibrated scale.
DC potentiometer, Wheatstone bridge, and other types of measurement

bridges use null-type instruments like a galvanometer as the null indicator
representing balanced conditions as a requirement of measurement of an

unknown quantity.
Accuracy of a deflection-type instrument depends upon the degree of

accuracy of the calibration. The accuracy of null-type instruments are higher
than deflection type of instruments because null indication means zero
current flowing through the instrument, and hence there is no effect of error

in measurement due to calibration. However, null-type instruments are not
suitable for the measurement of a quantity which changes with time.

11.8 MEASUREMENT ERROR

When we take the measurement of a certain quantity there may be some error

in the measurement due to a number of reasons. It is essential, therefore, to
know the causes of such error and find ways to reduce them. Study and
analysis of errors will determine the degree of accuracy of our measurements.

Measurement error can take place if the reading of the instrument is not
recorded correctly; if a large capacity instrument is used to measure a small

quantity, ie, because of improper selection of instruments; if the adjustment
of the instrument prior to measurement is not done (zero reading adjustment);
if there are changes in the environmental conditions like temperature,

electromagnetic interference, etc., during the measurement; if there are
defects in the instrument itself; if there are random variations in the

parameters or the system of measurement etc. Thus, we can see that errors
can be due to mistakes made by the person taking the measurement; due to

defects in the instrument or an improper use of instrument; due to changes in
the environmental conditions or changes in measurement parameters.

Errors are generally classified into three categories, namely, gross error,
systematic error, and random error.