Basic Terms : -
Before proceeding further I would like to discuss some basic terms.
True Value : -
Practically true value is the name given to the exact measurement of the quantity under observation. Technically it is the average value of an infinite number of measurement values when average deviation due to various contributing factors tends to zero. Such an ideal situation is impossible to realize in practice and hence it is not possible to determine the true value of the quantity by experimental means. The main reason for this is that the positive deviation from the true value donot equal to the negative deviation and hence do not cancel each other. Thus normally an experimental would never know that the value of quantity being measured by the experiment means is the true value or not. In fact, in practice the term true value than refers to a value that would be obtained if the quantity under consideration were measured by an exampular method i.e. a method agreed upon by expert as being sufficiently accurate for the purposes to which the data will ultimately be put to use.
Errors : -
As I told earlier 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 measurement.
A study of errors is a first step in finding ways to reduce them.
Types of errors : -
Errors are classified as : -
Gross Errors : -
The class of error mainly covers human mistake in reading instruments recording and calculating result.
The responsibility of the mistake normal lies with the experimental. The experimental may grossly misreal the scale. For example, he may, due to oversight, read 31.5 degree C implace of 35.1 degree C (actually reading ). Error in recording but as long as human being involvead, some gross errors will definility being committee. Although complete elimination of gross error is impossible, one should try to anticipate & correct them.
These can be avoided by two means :-
> Great care should be taken with reading & recording the data.
> 2, 3 or more reading should be taker for the quantity under measurement.
Systematic Errors : -
There are divided as :-
Instrumental :- These series fun to 3 main reason :-
Inherent short coming :- These are inherent in instrument because of their machinical structure. These may be due to construction, ealibration or operation of the instrument. These may cases of instrument to read too low or too high.
Example, If the spring used for producing controlling torque of a PM type instrument has become heat the instrument always read high. While making precision measurement we must recognise the possibility of such errors as it is often possible to eliminate them or atleast reduce them to a great extent by following methods :-
> Procedure of measurement must be carefully planned.
> Substitution method or calibration against standard may be used for the purpose.
> Correction factors should be applied after determining instrumental errors.
> Instrument may be recalibrated carefully.
Due to Misuse of Instrument : -
The errors caused in measurement are due to the fault of operator then that of the instrument. A good instrument used in an unintelligent way may give error.
Examples : -
Failure to adjust the zero of the instrument
Random Errors : -
These are the errors that are not common and occurs very rarely and normally the source of these errors are not known.