Every substance does not have the same conductivity level because they have different numbers of its free electrons
. This electrons' number depends on the smallest structure of the substance, which is atom. Even metals have different conductivity level, some of them provide better conductivity and the others provide average conductivity. This condition is caused by the resistance of the substance. Resistance is the opposition of the current flow. The unit of this resistance's measurement is called as OHM which is taken from the German professor George Ohm (1787-1854). The Resistance is symbolized as R while for OHM is omega (Greek symbol).