A rheostat is a variable resistor that regulates the electric current flow in a circuit by changing its resistance.
There are two things that determine the amount of current flowing into an electric circuit: the magnitude of the applied voltage and the overall resistance of that circuit. If the circuit resistance is lowered, the electric current flow that is passing through the circuit is boosted. Conversely, the electric current is limited if the circuit resistance is increased.
Putting a rheostat in a circuit allows its resistance to be changed, which in turn, either increases or decreases the current through the circuit. There is no need to connect separate resistors for increasing or decreasing the resistance, as a rheostat can perform both functions.
A rheostat has two connection points: one is a moving point and the other one is a fixed point. Similar to a potentiometer, some rheostats can have three connection points (A, B, and C) as shown in Figure 1, but still, only two of them are used. In such cases, there are two fixed points (A and C), only one of which is used, and the second connection point is the moving one (B).
Rheostats also have to bear large currents when compared with potentiometers. Therefore, rheostats are made up of wire wound resistors. Furthermore, rheostats may also be called variable wire wound resistors. They are mainly constructed by wrapping the Nichrome wire across a ceramic core. Such a core behaves as an insulator to the heat energy and does not permit it to flow through the rheostat.
Figure 1 explains the working principle of a rheostat. A rheostat works on the theory that the resistance of a certain track or wire depends upon its length. Let’s assume that we are using the fixed connection point A and the moving connection point B of the rheostat illustrated in Figure 1. The rheostat will offer minimum resistance to the circuit if the slider lies closer to point A, since the resistive coil length is at a minimum. Therefore, a large current can flow through the circuit in this case.
Similarly, the rheostat will offer maximum resistance if the slider lies closer to point C, as the resistive coil length is at its maximum. Hence, a tiny amount of current would flow through the circuit and a large part of the current would be opposed by the rheostat.
Now, let’s say that we are using the fixed connection point C and the moving connection point B. In this case, when the slider is made to lie near point C, the rheostat would offer minimum resistance and maximum current flow through the circuit. Likewise, when the slider is moved near to point A, the rheostat would offer maximum resistance and minimum current flow through the circuit.
Finally, it is important to know the maximum and minimum resistances necessary for your circuit. Rheostats have maximum and minimum resistance ratings, and thus, they cannot offer resistance outside their inherited range.
Rheostats are used in situations where high voltage is needed.
They are frequently used in light dimmers. When the rheostat amplifies its resistance, the electric current through the bulb is reduced and the light dims. Conversely if we decrease the rheostats resistance, the current will increase and the bulb will become brighter.
Additionally rheostats can be found anywhere that current needs to be increased or decreased. For instance they are commonly used in radios, to increase or decrease volume. Also they are often used to control electric motor speed. Lastly rheostats can be used in heaters or ovens to control temperature.