TOPIC TWO: MEASUREMENTS

SI UNITS: 

This is the System International of Units. They are specifically chosen units which have been agreed upon internationally to be used for measurements. 

BASIC QUANTITIES: 

These are quantities that can not be expressed in terms of other physical quantities. These quantities are sometimes referred to as Fundamental quantities and examples of the basic quantities with their SI units are given in the table below; 

DERIVED QUANTITIES: 

These are quantities that can be expressed in terms of other physical quantities. These quantities can be expressed in terms of fundamental quantities of mass, length and time and examples of the derived quantities with their SI units are given in the table below; 

Micrometer screw gauge: 

A micrometer screw gauge is used to measure very small lengths of about 0.01mm. e.g. measuring the diameter of a wire or measuring the thickness of paper. The length to measure using a micrometer screw gauge should be less than 1cm. 

How to read a Micrometer screw gauge:  

• Turn the thimble to tighten spindle very loosely on the object.  

• Turn the ratchet for final adjustment until it starts to click.  

• Read the last value on the sleeve scale.  

• Read the value of the mark on the thimble scale which coincides with the line on the sleeve scale to two decimal places.  

• Add that value to the value read on the sleeve scale which gives the final reading of the micrometer screw gauge. 

NOTE: When the anvil and spindle end are brought in contact, the edge of the circular scale should be at the zero of the sleeve (linear scale) and the zero of the circular scale should be opposite to the datum line of the sleeve. If the zero is not coinciding with the datum line, there will be a positive or negative zero error as shown in figure below. 

Metre rule: 

Metre rules are graduated in centimeters or millimeters. The following procedure should always be followed when using a metre rule 

i) Place the metre rule in contact with the object 

ii) Place the end of the object against the zero mark on the scale of the metre rule 

iii) Position your eye perpendicularly above the scale but not at an angle when reading the value 

Tape measure: 

There are several types of tape measures and these are tailor‟s tape measure, carpenter‟s tape measure and surveyor‟s tape measure. The choice of the tape measure to be used depends on the length to be measured and always ensure the tape measure is taut when measuring 

TIME: 

This is a measure of duration of an event. The SI unit of time is second (s). Other units are minutes, hours, days, weeks, months, years etc. 

Measurement of time: 

Intervals of time are measured using either a stop watch or a stop clock depending on the accuracy required. 

MASS: 

Mass is quantity of matter the body contains. The SI unit of mass is kilogram (kg). Other units are grams (g), milligram (mg) and tones 

The mass of an object depends on its size and the number of particles it contains therefore the mass of an object is the same everywhere because the number of particles in an object remains constant. That is an object has the same mass on the earth as on the moon. 

Measuring mass: 

The common instruments used for measuring mass are; 

i) Top pan balance. 

ii) Beam balance. 

iii) Lever balance. 

iv) Weighing scale. 

v) Spring balance. 

Measuring volume of liquids:  

Liquids have no definite shape but assume the shapes of the containers in which they are put. The following instruments are used in measuring the volume of liquids and these are measuring cylinder, pipette and burette, conical flasks, beakers, round bottom flasks e.tc. Measuring cylinders are made of glass or transparent plastic and graduated in cm3 or ml and measuring flasks, pipettes, burettes and beakers are used to transfer known volumes of liquids. 

Measuring the volume of an irregular object: 

Volumes of irregular solids are measured using the displacement method. The method works with solids that are not soluble in water i.e. do not absorb water or react with water.