FORM 4: 1.4 UNDERSTANDING MEASUREMENTS

THE MEANING OF CONSISTENCY

The consistency of a measuring instrument is its ability to register the same reading when a measurement is made repeatedly.

THE MEANING OF ACCURACY

Accuracy of a measurement is how close the value of the measurement to the actual value. 

                          

THE MEANING OF SENSITIVITY

Sensitivity of an instrument is its ability to detect a small change in the quantity to be measured.

MEASURING INSTRUMENTS	SMALLEST MEASUREMENT (cm)	DEGREE OF SENSITIVITY
metre rule	0.1	low
vernier callipers	0.01	medium
micrometer screw gauge	0.001	high

THE MEANING OF ERROR

Error is the difference between the actual value of a quantity and the value obtained in measurement.

THE MEANING OF SYSTEMATIC ERROR

Systematic error is the error that will produce the reading that is always higher or always smaller than the actual reading.

SYSTEMATIC ERRORS ARE DUE TO

i)                    error in calibration of instrument

ii)                   zero error of the instrument

iii)                 error in reaction time

iv)                 wrong assumption in measurement

THE MEANING OF RANDOM ERROR

Random error is the error that will produce inconsistent readings due to the carelessness of the observer in making a measurement.

RANDOM ERRORS ARE DUE TO

i)                    parallax error

ii)                   sudden change in ambient factors such as temperature and air circulation

HOW TO AVOID RANDOM ERRORS

Repeat the measurements several times and take the mean value.

THE MEANING OF ZERO ERROR

Zero error is the systematic error that occurs when the pointer of the instrument does not return to zero when not in use.

THE MEANING OF PARALLAX ERROR

Parallax error is the random error that occurs due to the incorrect positioning of the eyes in taking reading.

HOW TO AVOID PARALLAX ERRORS

Line of view must be normal to the plane of the scale when taking readings.

VERNIER CALLIPERS

HOW TO TAKE THE READING OF VERNIER CALLIPERS

i)                    Read the mark on the main scale proceeding the ‘0’ mark on the vernier scale.

ii)                   Read the mark on the vernier scale that is exactly in line with any mark on the main scale.

MICROMETER SCREW GAUGE

HOW TO TAKE THE READING OF MICROMETER SCREW GAUGE

i)                    Read the main scale reading at the edge of the thimble.

ii)                   Read the thimble scale reading at the horizontal reference line.