Introduction To Units And Measurement

Measurement

Meter
Meter is the unit of length in S.I. System.
Meter is defined as “The distance between the two marks on a Platinum-Iridium bar kept at 0OC in the International Bureau of Weight and Measures in Paris.”
One meter = 100 cm
One meter = 1000 mm

Kilogram
Kilogram is the unit of mass in S.I. System.
“Kilogram is defined as the mass of a platinum cylinder placed in the International Bureau of Weight and Measures in Paris.”
One kilogram = 1000gram

Second
Second is the unit of time in S.I. System.
A second is defined in terms of the time period of Cs-133 atoms. i.e.” one second is equal to 9,192,631,770 periods of vibrations of Cs-133 atoms.”
60 seconds = one minute
3600 seconds = one hour

Least Count
Minimum measurement that can be made by a measuring device is known as ” least count’.
Least count (vernier callipers) = minimum measurement on main scale / total number of divisions on vernier scale

Least count (screw gauge) = minimum measurement on main scale / total number of divisions on circular scale
Smaller is the magnitude of the least count of a measuring instrument, more precise the measuring instrument is.
A measuring instrument can not measure anything whose dimensions are less than the magnitude of the least count.
Least Count of Vernier Callipers = 0.01 cm
Least Count of Micrometer Screw gauge = 0.001 cm

Zero error
It is a defect in a measuring device (Vernier Callipers & Screw Gauge).
When jaws of a Vernier Callipers or Screw Gauge are closed, zero of main scale must coincide with the zero of vernier scale or circular scale in case of screw gauge.
If they do not coincide then it is said that a zero error is present in the instrument.

Types of zero error
Zero error may be positive or negative. A positive zero error in the instrument shows a larger measurement than the actual measurement. In order to get exact measurement, positive zero error is subtracted from the total reading.
A negative zero error in the instrument shows a smaller measurement than the actual measurement. In order to get exact measurement, negative zero error is added to the total reading.

Pitch
“Perpendicular distance between two consecutive threads
of the screw gauge or spherometer is called PITCH.”
Pitch = Distance traveled on main scale / total number of rotations.

Error
An error is defined as “The difference between the measured value and actual value.”
If two persons use the same instrument for measurement for finding the same measurement, it is not essential that they may get the same results. There may arises a difference between their measurements. This difference is referred to as an “error”.

Types of error
Errors can be divided into three categories:

1. Personal Error
2. Systematic Error
3. Random Error

1. Personal error
An error comes into play because of faulty procedure adopted by the observer is called personal error. Personal error comes into existence due to making an error in reading a scale. It is due to faulty procedure adopted by the person making measurement.

2. Systematic error
The type of error arises due to defect in the measuring device is known as systematic error. Generally it is called “zero error”. It may be positive or negative error. Systematic error can be removed by correcting measurement device.

3. Random error
The error produced due to sudden change in experimental conditions is called random error.
For example:
Sudden change in temperature, change in humidity, fluctuation in potential difference (voltage).
It is an accidental error and is beyond the control of the person making measurement.