What is Rheostat?
A rheostat is an electronic instrument that is used for the purpose of controlling the electric current which is flowing through a circuit. Thus, it is basically a variable resistor that exhibits controlling action.
The name for this instrument was given by an English scientist Sir Charles Wheatstone.
Basics of Rheostat
In any electric circuit whenever there is a need for adjusting the current which is flowing through the circuit then rheostats are used. It is mainly used for changing the resistance of the electric circuit. Generally, the resistive element of this instrument is a metal wire or it can be carbon or a conductive liquid.
In the circuits where a small amount of current flows and is required to be controlled then carbon type of resistive element is used. While in case the current flowing through the circuit is large then the electrolytic type of resistive elements are taken into consideration where the rods are placed within the conducting fluid.
Thus, we can say that rheostats are the variable resistors that are designed to provide resistance to the circuit.
It is known to us that the current which flows through any electric circuit depends on either the amount of supply voltage provided to it or the overall resistance of the electric circuit.
The resistance of the circuit when increased resultantly causes a reduction in the current which is flowing through the circuit. As against, if the resistance is reduced then the flow of current will automatically increase.
Rheostat when placed in an electrical circuit can control the flow of current through the circuit as these can increase or decrease the current. One should note here that rheostats cannot fully block the electric current flowing through the circuit, it can only limit the flow of current as practically it is not possible to do so.
Symbolically rheostats are represented as:
As we can see there are two representations, in one there are zigzag lines which are known to be American standard while in another there is the rectangular box which is the international standard.
Classification of Rheostats
Rheostats are mainly classified into two categories:
- Rotatory Rheostats: Also given the name circular rheostat as the resistive element over here is present in a circular orientation. Basically, the motion of the slider, in this case, is rotational. These are more preferred than linear rheostats as they are compact.
- Linear Rheostats: Another name for this is cylindrical rheostat and the reason is that the resistive element present here is of cylindrical orientation. However, the slider here moves in a linear fashion.
Working of Rheostat
The operating principle of rheostat is based on Ohm’s law, according to which the current varies in inverse proportion to the resistance for the supplied voltage. This implies that the current decreases if resistance decreases and vice-versa.
A rheostat is known to be a two-terminal device and out of the two one is fixed while the other is mobile.
For any electric circuit, it is universally known that if one wants to vary the current then either applied voltage or resistance through the circuit must be changed. The incorporation of rheostats in electric circuits enables the variation in resistance in order to change the current.
It is known to us that current and resistance vary in inverse proportion with respect to each other and therefore if the current is required to be decreased then the resistance of the rheostat must be increased. While opposite action is performed whenever there is a need to increase the current through the circuit.
At the time of increasing or decreasing the resistance of rheostat, it is to be kept in mind that it can be increased or decreased up to a certain limit and this depends on the resistance rating of the rheostat.
Suppose there is a rheostat of rating 40 kΩ then the lowest resistance which it can offer is 0 Ω and the highest is 40 kΩ.
It is the fact of basic understanding that the resistance exhibited by any material shows dependency on the length, cross-sectional area along with resistivity of the material which is considered.
Thus, whenever there is a need to change the resistance then the effective length is varied through sliding contact. The effective length is the length between the fixed terminal and the point where the sliding terminal is present. With the motion of the slider, the effective length gets varied and this causes a change in the resistance of the rheostat.
R = ρl/A
Since length varies in direct proportion to the resistance, therefore, if the length is increased then resistance also increases and vice-versa.
Various applications of rheostats are due to their ability to control current. In any circuit current controlling is necessary to prevent the damage due to high current faults, in such cases, rheostats are used. In circuit tuning as well as calibration, rheostats are used.