By the middle of the eighteenth century, electrostatics was a well established branch of physics. Several experiments had been conducted and many machines built that would produce through friction great amounts of charge giving sparks and electric shock. Electric shock was generated due to a momentary flow of charge. This flow of charge is what is referred to as electric current.
It should be noted that all matter in the universe is composed of tiny units called atoms. An atom is composed of a dense nucleus and surrounded by an electron cloud or shell called an electron orbit. The nucleus of an atom contains dense positively charged protons and electrically neutral neutrons. The electrons are bound to the nucleus by an electromagnetic force. In any material atoms are bound together forming a molecule. An atom may be electrically neutral if the amount of charge of protons in the nucleus balances the charge on the electrons; otherwise it is regarded as a cation or anion. A cation has an excess of positive (protons) charge while an anion has an excess of negative (electrons) charge.
Atoms are classified according to the number of protons and electrons that they have. The smallest atom is hydrogen with a proton in the nucleus and a single electron orbiting the nucleus under the influence of an electrostatic force. This force is inversely proportional to the size of the atom. Hence, in large atoms, the outer electrons are loosely bound and can easily be lost or given when such an element reacts. For any element to react it must lose (case of metals) or gain electrons (case of non metals).
Electrons in an atom are arranged in charge clouds or shells around an atomic nucleus called orbits. The outermost electrons participate in bonding and all chemical reactions.
In metals, atoms lose their outermost electrons to form metal cations. The electrons from all the metal atoms form a sea of delocalized electrons that can flow around these metal cations. The electrons are released in a pool within the lattice and can move freely through the lattice. When a potential difference is connected across this lattice an electric field is created that re-aligns the electrons in the direction of the electric field and hence electrons begin to drift in the direction of the field constituting what is known as an electric current.
In ionic lattices, electrons are transferred from one atom to another resulting in the formation of positive and negative ions. The electrostatic attraction between the positive and negative ions holds them together forming an ionic lattice. Take for instance: Sodium chloride is an ionic compound. In solid state, Sodium chloride does not conduct electricity except when in molten form or when dissolved in water. When a potential difference is connected across two electrodes dipped in a solution of sodium chloride, an electric field is set up between them that realigns the ions. The negative ions are urged to the anode while the positive ions are urged to the cathode. The drift of anions from cathode to the anode constitutes an electric current.
Generally, Ionic compounds are good conductors of electricity when melted or dissolved in water because the ions dissociate in these states and become free to move and carry electrical current. In the solid state, the ions are not free to move, and the solid ionic compound cannot carry electricity.
All in all, an electric current will flow for as long as there are free electrons or freely moving ions. The drift of these electrons or ions will constitute an electric current.