All the substances are made up of atoms which consist of electrons. Metals have some loosely bound electrons which could be detached temporarily. On detachment, they could also be moved within the metal if an influence is given by an EMF. The flow of electrons is electric current.
The movement of electrons happens easily in copper, aluminium, silver, etc. These materials are called conductors. A material that does not have free electrons will not permit electron flow. It is called an insulator or dielectric e.g. porcelain, rubber, mica etc. Materials which have some characteristics that belong to neither of the other groups are called semiconductors. E.g. silicon.
Resistance
Resistance is the property of any substance due to which it resists (opposes) the flow of current in it. The unit of resistance is Ohm. A conductor is said to have a resistance of one ohm if it permits one ampere current to flow through it when one volt is impressed across its terminals. The media given below defines the resistance and change in it due to change in the conductor size.
Thermistor
A thermistor is made of metallic oxides in a binder. It has a large negative temperature coefficient of resistance. Its resistance decreases with increase in temperature as shown in the image given below.
Varistors
It may be seen that the current through a varistor increases rapidly when the applied voltage increases beyond a certain amount. This means that there is a corresponding rapid decrease in resistance when the current increases.
Construction of Varistors
Varistors made of metal oxides are mostly used to protect against high voltage surges caused in the transmission and distribution systems by lightning where the voltage surge happens for microseconds.
The lightning current could be discharged in microseconds if resistance in the discharge path is reduced in time. Varistors are capable of giving this reduced resistance value when the voltage increases. By discharging the surges thus, the interruption in the electricity system is averted.
