Matter is made up of protons, electrons and neutrons. Protons carry positive charge, electrons carry negative charge and neutrons do not carry any charge. An atom is electrically neutral but if a body carries excess of protons than the electrons, the body gets positively charge. If the body has excess of electrons than the protons, body gets negatively charged.
If a charged body is connected to an uncharged body through a metallic wire, the positive charge flows from higher potential to lower potential while negative charge flows from lower potential to higher potential. The charge flows till both the bodies are at the same potential.
To pass the charge continuously from one body to another body through a wire a constant potential difference has to be maintained between the two ends of a wire in a circuit. This is done by an external source of energy which forces the charge carriers (electrons) already present in the wire to move in a definite direction i.e. from lower potential region to higher potential region. The external source of energy is called a cell.
A cell is a device in which chemical energy is converted into electrical energy. In the cell, negatively charged plate repels the electrons which causes the electrons to move along the wire. Hence the electrons flow from the negatively charged plate through the wire to positively charged plate of the cell. This is known as the electron current.
Conventionally the direction of the current is taken as opposite to the direction of the flow of electrons i.e., from the positive to the negative terminal.
The combination of cells is called a battery.
One of the earliest and simplest devices capable of producing steady current was invented by Alessandro Volta (1745-1827) named Voltaic Cell. Batteries are a good source of Direct current. Direct current (DC) means the electric current is flowing in one direction only in a circuit. To measure the current in a circuit, ammeter is used.