Physical Quantities and Measuring (part 2)
Table of content:
|Mass Measuring Instruments
However, an accuracy greater than 1 mm obtain by using some other instruments such as Vernier Callipers.
Parts of Vernier Callipers:
They consists of two parts. First of all describe the first part of Vernier Callipers one is a fixed jaws with main scale attached to it. Fixed jaws is also place on main scales. Main scale has lines in centimetre and millimetres marks on it. Second part of Vernier Callipers is that moveable jaws. It is call Vernier scales. It has Vernier scale having 10 divisions place on the moveable jaws such that each of its divided into 0.9 mm.
Least Count of Vernier calliper:
The difference between one small division on main scale division and one Vernier scale division is 0.1mm. It is know as least count of the Vernier calliper. Least count of Vernier calliper can also written as below:
L. C of Vernier callipers= smallest reading on main scale / no. of division on Vernier scale
Least count of Vernier calliper= 1 mm / 10 division = 0.1 mm
L C= 0.1 mm = 0.01cm
Working of a Vernier callipers:
First of all find the error on Vernier calliper. If any in the measuring instrument. It is says that zero error of the instruments. Knowing the zero error, necessary correction can be made to find the correct measurement. Such a correction is know as zero correction of the instrument. Zero correction is the negative then zero error.
Zero Error and Zero Correction:
To find the zero error, First of all fixed jaws(main scales) and moveable jaws( Vernier scales) meet to gather. If zero line of the Vernier scale with the zero of the main scale. Then the zero error is zero. Zero error will exist if zero line of the Vernier scale is not coinciding with the zero of the main scale.
Zero error will be positive if the zero line of the Vernier scale is on the right side of the zero of the main scale.
Zero error will be negative if the zero line of the Vernier scale is on left side of the zero of the main scale.
A screw gauge is an instrument that is used to measure small lengths with accuracy greater than a Vernier callipers. It is also says that micrometres screw gauge. A simple screw gauge consists of a U shape metal frame with a metal stud at its one end.
A hollow cylinder has a millimetre scale over it along a line called index line parallel to its axis.
The hollow cylinder act as a nut. Its fix at the end of U shape frame opposite to the stud. A thimble has a threaded spindle inside it. As the thimble completes one rotation, the spindle moves 1 mm along the index line. It is because the distance between consecutive thread on the spindle is 1 mm. This distance is know as the pitch of screw on the spindle. The thimble has 100 division around its one end. It is the circular scale of the screw gauge. As thimble is completes one rotation, 100 division pass the index line and thimble moves the 1 mm along the main scale. Thus each division of circular scale crossing the index line moves the thimble through 1/100 mm or 0.01mm on the main scale.
Least count of screw gauge:
Small reading that can measure using any instrument is called least count. Least count of screw gauge is equal to the pitch of the screw gauge is divided by no of divisions on circular scale.
Least count= pitch of the screw gauge / no. of divisions on circular scale
L. C=1mm/ 100=0.01mm=0.001cm
Thus the least of the screw gauge is 0.01mm or 0.001cm.
Working of a screw gauge:
First of all, to find the zero error of the screw gauge.
To find the zero error, close the gap between the spindle and the stud of the screw gauge by rotating the ratchet in the clockwise direction. If zero of circular scale meet with the index line, then the zero error will be zero.
positive Zero Error :
Zero error well be positive if zero of circular scale is behind the index line.
Negative Zero Error :
Zero error will be negative if the zero of circular scale has the cross the index line.
Mass Measuring Instruments:
Instruments used to measure mass of various object are called Mass Measuring Instruments.
Beam balance is used to measure the mass of various object by comparison.
A physical balance is used in the laboratory to measure the mass of various object by comparison. It consists of a beam resting at the centre on a fulcrum.
A lever balance consists of a system of levers. When lever is lifted placing the object in one pan and standard masses on the other pan, the pointer of the lever system moves. The pointer is brought to zero by varying standard masses.
Electronic balance such as come in various ranges, milligram ranges, gram ranges and kilogramme ranges. Before measuring the mass of a body, it is switched ON and its reading is set to zero. Next place the object to be weighed. The reading of the balance gives you the mass of the body placed over it.
A measuring the cylinder is a glass or transparent plastic cylinder. It has a scale along its length that indicate the volume in millilitre. Measuring cylinder have different capacities from 100ml to 2500 ml. They are used to measure the volume of a liquid or powdered substance.
How to use a measuring cylinder:
Take a measuring the cylinder. Place it vertically on the table. pour some water into it. The correct method to note the level of a liquid in the cylinder is to keep the eye at the same level as the meniscus of the liquid.
The accuracy in measuring a physical quantity depends upon various factors;
1. The quality of the measuring instrument.
2. The skill of the observer.
3. The number of observations made.
The following rules are helpful in identifying significant figure;
(i) Non zero digits are always significant.
(ii) Zero between two significant are also significant.
(iii) Final or ending zero on the right in decimal fraction are significant.
(iv) Zero written on the left side of the decimal point for the purpose of spacing the decimal point are not significant.
(v) If number are written in some power 10. Then all the number before power of ten are significant.
Rounding the number:
Droping the insignificant figure or digits from measured value is called rounding the number.