# SI of measurements

### The International System of Units

Quantitative measurement is the cornerstone of modern science, but it has not always been so.  Quantitative measurement was developed for other purposes, as technology, and was only then adopted for scientific use. The system of weights and measures were developed on an ad hoc basis in different parts of the world. The most fundamental quantities measured were mass or weight, length or distance, and time. Systems of units for measuring these were developed from the very beginning of recorded history. Measurement of temperature was added in the sixteenth century, and measurement of electric current in the eighteenth century. More recently the amount of substance and luminous intensity have been added in the International System of Units, or SI.The International System of Units or Systeme Internationale (SI) is an improved metric system adopted by the Eleventh General Conference of Weights and Measures in 1960. It is the universal measuring system used in all areas of science throughout the world. The entire SI system of measurement is constructed from seven base units, each of which represents a single physical quantity as shown in the table below.

### Base Units of the International System

Quantity
Name of Unit
Unit Symbol
length
metre
m
mass
kilogram
kg
time
second
s
temperature
kelvin
K
amount of substance
mole
mol
electric current
ampere
A
luminous intensity
candela
cd

The metric system is a system of measuring. It has three main units:

 m the meter for length kg the kilogram for mass s the second for time
With those three simple measurements
we can measure nearly everything in the world!

## Examples:

### Meter

 The length of this guitar is about 1 meter: When unfolded this ruler measures 2 meters:

### Kilogram

 This gold bar has a mass of 1 kilogram. A dictionary also has a mass of about 1 kilogram.

### Second

1 second is about as long as it takes
to say “one thousand and one”

## Larger or Smaller

But what if we want to talk about really big or really small things?

Answer: we can use Metric Number Prefixes

• like “kilo” (a thousand)
• and “milli” (one thousandth)
• and so on

### Examples:

something that is 1,000 meters is a “kilometer”

a very short time of one thousandth of a second is a “millisecond”

In fact the kilogram already uses this method, as it’s a thousand grams, a kilogram.

Here is a quick summary of the special prefixes:

## Common Big and Small Numbers

 Name The Number Prefix Symbol trillion 1,000,000,000,000 tera T billion 1,000,000,000 giga G million 1,000,000 mega M thousand 1,000 kilo k hundred 100 hecto h ten 10 deka da unit 1 tenth 0.1 deci d hundredth 0.01 centi c thousandth 0.001 milli m millionth 0.000 001 micro µ billionth 0.000 000 001 nano n trillionth 0.000 000 000 001 pico p

### Quiz

Awareness of matter