**Rules for Writing Symbols of Units**

1. Symbols for a unit named after a scientist should have a capital letter.

Eg: N for newton, W for watt, A for ampere.

2. Full names of the units, even they are named after a scientist should not be written with a capital letter.

Eg: newton, watt, ampere, metre.

3. Symbols should be written either in full or in agreed symbols only.

4. Symbols for units do not take plural form

Eg: 100 Hz but not 100 Hzs.

20 V but not 20 Vs

2 A but not 2 As

**Multiplication factors of units:**

The multiples and sub-multiples of a unit are written by adding suitable multiplication factor as prefix to the unit. The names and symbols of various multiplication factors are shown in table below.

Multiplication Factor | Name | Symbol |

10 ^{18} | exa | E |

10 ^{15} | peta | P |

10 ^{12} | tera | T |

10 ^{9} | giga | G |

10 ^{6} | mega | M |

10 ^{3} | kilo | K |

10 ^{2} | hecto | H |

10 | deca | da |

10 ^{-1} | deci | D |

10 ^{-2} | centi | c |

10 ^{-3} | milli | m |

10 ^{-6} | micro | m |

10 ^{-9} | nano | n |

10 ^{-12} | pico | p |

10 ^{-15} | femto | f |

10 ^{-18} | atto | a |

**DEFINITIONS:**

Instrument: A device for determining the value or magnitude of a variable or quantity. An electronic instrument works on electrical or electronic principles for its measurements.

Accuracy: It is the closeness to which an instrument can read the true or actual value of the quantity or variable being measured.

Precession: It is the degree of reproducibility among several independent measurements of the same true value under reference conditions.

Sensitivity: It is the ratio of the output given by the instrument or its response to a change of input signal or variable being measured.

Resolution: It is the smallest change in the value measured for which the measuring instrument will respond.

Error: It is the amount of deviation from the true value of the variable or quantity being measured.

**TYPES OF ERRORS :**

When the indicated value of the instrument is more than the true value the error is said to be positive. In converse if the indicated value of the instrument is less than the true value the error is said to be negative.

1. Gross errors: Mostly these are human errors due to wrong reading of instruments, improper adjustment, application of the instruments and mistakes in computation.

2. Systematic errors: These errors are due to the wear and tear, aging of parts. Equipment also will be affected by environment.

3. Random errors: These are due to variations in the parameter or system at random that cannot be directly established.

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