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Conductors and Insulators
All substances have some ability to transmit electrons but they differ greatly in the ease with which electrons pass through them. For instance, a copper wire conducts electricity readily, glass conducts so little currents that it is hardly measurable. Substances through which currents easily pass are the conductors. Substances strongly resisting the flow of current are the insulators. An insulator is also called a dielectric.
Almost all metals are good conductors of electricity but we consider silver to be the best conductor of all. Copper comes next, it is followed by aluminium. Copper is our commonly used conductor.
Some liquids also conduct electric currents. They even prove to be good conductors of electricity. Most gases conduct current under proper conditions of pressure and temperature. However, they are not as good electrical conductors as metals.
The most common materials used to resist electric flow are: glass, rubber, porcelain, paper, oil, cotton and silk. Non-ionized clean air is also considered to be a good insulator. It is often used for this purpose in electrical apparatus.
It would be quite wrong to think that conducting materials are the only materials used for the transmission of power. We need both conductors and insulators. We cannot do without the copper wire which conducts the electric power and, acts as a conductor. However, we have to use an insulator to prevent electrical losses.
In the transmission of power we generally employ the best conductors.

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