Rheostat – Working One of the most omnipresent electric components is the resistor. In applications where variable Resistance is required, potentiometers and Rheostat are mostly preferred. We have already discussed about potentiometers in our previous article about the same. Here we shall discuss about rheostat in detail. What is a Rheostat? Rheostat is a type of variable resistor, whose resistance can be changed so as to change the amount of current flowing through a circuit. This device, was named “Rheostat” using two Greek words “rheos” and “statis” (meaning a current controlling device), by an English Scientist Sir Charles. It has two terminals, out of which one is fixed and the other one is a moving terminal. Some rheostats have three terminals just like potentiometer, although only two terminals are used (Only one of the two fixed terminals and the moving terminal are used). Some practical rheostats are shown below.   Unlike potentiometers, these devices have to carry significant amount of current. Hence, wire wound resistors are mostly used for the construction of rheostats. In a circuit diagram, a rheostat is often represented as shown below.     So on what basis does the rheostat work? Let’s get to know that in the next section. Working principle of a Rheostat In order to understand, the significance of rheostat and its working principle, let us refresh our basic of electric circuits. The three main parameters of an electric circuit are: voltage applied to the circuit, current through the circuit and the resistance offered by the circuit. Now, we know these parameters are interdependent. That is, to change the current, we can either change the voltage applied or change the resistance of the circuit. When we use a rheostat in a circuit, what we are basically doing is changing the resistance of the circuit, to change the current. As the current and resistance are inversely proportional, if a decrease in current is required, we will increase the resistance of the rheostat. Similarly, if an increase in current is required we would simply decrease the resistance of the rheostat. Now you may wonder whether there is there a maximum limit up to which the resistance can be decreased or increased in a rheostat. The answer is yes, there is. For every rheostat comes with a resistance rating, like for example if a rheostat has a rating of 50KΩ, the minimum resistance it can offer is 0 and maximum is 50kΩ. So how do we change the resistance of the rheostat? For this, rewind your basics of resistance. In our previous article “Resistivity and Electrical Conductivity – Complete Guide”, we have discussed the parameters on which resistance of a material depends.  The main three factors on which the resistance of a material depends is its length, area of cross-section and the type. Here, in this device, the effective length is changed using a sliding contact. A rheostat as already mentioned has a fixed and a moving terminal. The effective length is that between the fixed terminal and the position of the sliding terminal on the resistive path.  As the slider moves, the effective length changes thereby changing the resistance of the rheostat. Since resistance is directly proportional to the length, as the effective length increases, the resistance increases. Similarly, as the effective length decreases, resistance offered by the rheostat decreases. Now that the working principle is quite clear, let’s see, the construction and types of rheostats. Construction of Rheostat: The construction of rheostat is same as that of a potentiometer, as discussed in detail in our article about potentiometers.  Similar to the potentiometer, a rheostat... read more

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