For burning a lamp, for running a fan or for giving supply to the armature of a d.c. motor, two wires are necessary. Through one wire the current enters into the machine or apparatus. This wire is called positive (+) wire. Through the other wire the current returns back to the supply source. This wire is called negative (-) wire.
Thus, the path of the flow of current is completed through two wires and this path is called electric circuit. For the flow of current, electric circuit must be a closed one. If at any point in the circuit the wire is cut or broken or the connection of wire is open, the circuit is called an open circuit. Electric current cannot flow through such a circuit.
Thus, an electric circuit is complete with two wires. This is shown in fig. 74 in which the current is coming out of a generator and flowing towards a lamp through the positive wire. From the lamp the current is flowing back to the generator again through the negative wire.
The circuit may also be complete through one wire only. This is shown in fig. 75. In this circuit positive wire is drawn from the positive terminal of the generator up to one terminal of the lamp. The negative terminal of the generator and the other terminal of the lamp are connected to earth through water pipe or through some other form of earth connections.
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The current flows from the generator to the lamp through the positive wire, but returns back to the generator through earth. This is called earth return. For house wiring or motor wiring, such type of connection is not usually adopted. Here both the positive and the negative wires are drawn in the circuit and these wires must be insulated. In low voltage circuits, such as telegraph circuit or the circuit of a calling bell, sometimes earth return system is followed in order to reduce the cost of wiring.
