Rheostat

The rheostat is an electronic device that is used to vary the electric resistance without in interrupting the electric circuit.   Most of the people might be familiar with the rheostat in the form of slider or dimmer that is used vary the intensity of the light.

Rheostat is a device to control electric current in a circuit. It consists of a number of turns of an oxidised nichrome wire, having high resistivity and neglgible temperature coefficient of resistivity, wound on a hollow cylinder of china-clay. These turns are insulated from one another due to being oxidised. The two ends of wire are joined to connecting screws A and B. A metallic rod CD, carrying a sliding strip, is held parallel to cylinder. This strip can be moved to and fro and touches the coil with pressure. It is called sliding contact. One end of the rod carries a connecting screw C. A and 5 are called fixed ends and C is called the variable contact. By the help of three screws A, B and C; a rheostat can be used in two ways:

• As a current controller: To control electric current in a circuit; rheostat is connected in series with the circuit. For this the two terminals of circuit are connected to a fixed end A (or B) and a sliding contact C. When electric circuit is connected to screws A and C, the length AX of coil in circuit. When electric circuit is connected to screws B and C, the length XB of coil is included in circuit On moving the sliding contact to and fro on the rod CD, the resistance in circuit may be changed which causes change in current in circuit
• As a Potential Divider: In some experiments it is desired to obtain variable potential difference with a single battery. For this we use a rheostat as a potential divider. For this the battery is connected across fixed terminals A and B. The two terminals A and C (or B and Q are called the output terminals. These terminals A and C (or B and C) are connected the circuit where variable potential difference is to be supplied.

Questionnaire:

• What is rheostat? Explain.