We all must have witnessed and used a cylindrical device called the rheostat while performing experiments in the Physics lab. But we never really delved into its technicalities. Electrical resistance is a fundamental property, and rheostats are the key to harnessing it.
This article will explore the history, principles, applications, and different types of rheostats, highlighting their significance in modern technology.
A rheostat is a type of variable resistor, whose resistance can be changed for varying the amount of electric current flowing through an electrical circuit. Commonly available resistors have a fixed value and are used to restrict smaller electric current values. Rheostat is used for varying higher values of electric current.
Brief History
In the nineteenth century, Sir Charles Wheatstone invented the rheostat using a long tube with coiled wires around it and an adjustable slider. The word rheostat is made of two words (‘rheo’ meaning flow of current in Greek and ‘stat’ meaning stationary instrument).
When placed in an electric circuit, the flow of electricity changed through two terminals: one terminal near the sliding contact and the other connected near the bottom.
Rheostat Construction
The modern-day rheostat is not much different from its earlier version. A long cylindrical structure having a ceramic core has Nichrome wires tightly wound around it. The ceramic core acts as an insulating material to the generated heat.
Similar to a potentiometer, the rheostat has three terminals out of which only two are used. A slider is present at the top, which can freely move and is in contact with the wire wound wires.
Rheostat Working Principle
The working of rheostat is based on Ohm’s law, which is given by:
R = V/I
Where,
- R= resistance
- V= voltage
- I= current
From the above law, we can see that resistance is inversely proportional to current. This means that an increase in resistance decreases the current and vice-versa.
Also, according to the following formula:
R = ρL/A
Where,
- R = resistance
- ρ = resistivity
- L = length
- A = area of cross-section
resistance is directly proportional to length. Therefore, resistance increases as the length of the wire (i.e. number of turns) increases.
Rheostat Connections
As stated earlier, out of three terminals of a rheostat, only two are used.
The above diagram shows how connections are made in a rheostat when placed in an electrical circuit. One end of the wire from where the current enters the device is connected to the bottom-left terminal (terminal A).
By moving the wiper/slider, the resistance can be increased or decreased. This varied current then flows out through the top-right terminal (terminal B) further into the electrical circuit.
The wiper/slider being close to terminal A indicates low resistance whereas it increases when close to terminal B.
If we use terminals B and C, a minimum resistance is achieved when we move the wiper/slider close to terminal B because the length of the resistive path is now decreased. This leads to a flow of large amounts of electric current.
When the wiper/slider is moved to terminal A, maximum resistance is achieved as the length of the resistive path increases. Therefore, a large flow of electric current gets restricted.
Rheostat Symbol
A rheostat is internationally denoted by the following symbol:
Rheostat Applications
Rheostats have numerous daily life applications that extend far beyond lab experiments. Rheostat is generally used in applications where high voltage or current is required such as:
- Changing the light intensity of a light bulb: An increase in the resistance of the rheostat decreases the flow of electric current, leading to dimming of lights and vice-versa.
- Generators: Rheostat adjusts the resistance, controlling the excitation current that affects the magnetic field strength. This in turn regulates the output voltage. o regulate excitation voltage to control output current and voltage, we use rheostat
- Motor speed: Rheostat adjusts the motor speed by controlling the armature current.
- Heater and oven temperature control: Rheostat maintains a set temperature by adjusting the heat output by controlling the heater current. When we connect to a temperature sensor, we can detect excessive temperature, and then the rheostat triggers the relay/switch that activates the cooling system, or it can turn off the power source.
- Volume control: Rheostat when connected to an audio signal can control the signal amplitude by varying the resistance. This change in signal amplitude affects the volume.
Types of Rheostat
- Linear: It is cylindrical in which the wiper or slider moves linearly. It has a linear resistive path. These are mostly used in laboratories for teaching and experiments.
- Rotary: It has a rotary resistive path. In this, the wiper or slider is mounted on a shaft and moved in a rotary manner, rotating over 3⁄4 of a circle. These are mostly used in power applications.
- Preset: These are small in size and are nothing but a small rheostat. Trimmers or preset rheostats are used in a printed circuit board for calibration purposes.
Difference Between Rheostat and Potentiometer
Although rheostat and potentiometer serve the purpose of varying the amount of resistance, they have certain dissimilarities.
| Rheostat | Potentiometer |
| 2-terminal device; two terminals used for the operation | 3-terminal device; three terminals used for the operation |
| Cannot be utilized like a potentiometer | Can be utilized like a rheostat |
| Often used for high-power applications (Motors, heaters, etc.) | Often used for low-power applications (Sensor measurement, audio volume control) |
| Used for controlling current | Used for voltage division, measurement, and control |
Common Errors and Solutions
| Common Errors | Solutions |
| No Change in Resistance | → Verify the rheostat connections and wiring. → Ensure that the wiper is not stuck or damaged, and it can move freely. |
| Wiper Gets Stuck | → Lubricate the shaft and wiper mechanism if they are jammed. → Clean/replace the wipers regularly → Check If the wiper is bent or damaged |
| Rheostat Not Responding | → Check for any loose connections → Ensure the resistance range matches the application |
| Rheostat Overheating | → The rheostat may be overloaded. Keep an eye out for power rating and use a rheostat with higher wattage if needed |
Can rheostat be used as a switch?
Yes. But ideally, rheostats are designed for varying the resistance
How do you select a suitable rheostat?
Consider factors like power rating, resistance range, and application requirements
Can we use rheostat for both AC and DC circuits?
Yes, we can use rheostat for both AC and DC circuits. However, it is preferred to use rheostat for DC applications. This is because the inductive and capacitive effects of AC can affect the rheostat behaviour.
Can I use rheostat for voltage amplification?
No. Rheostat is used for voltage division not amplification.
Give an example of where a rheostat is used
A rheostat is used in applications requiring adjustable resistance, like light dimmers, motor speed controls, heaters, and testing equipment. By varying resistance, it controls current flow to adjust brightness, motor speed, temperature, or simulate loads in testing setups.
Why do we include a rheostat in the circuit
To provide variable resistance, allowing for fine control over current flow. This is essential in applications where adjusting the intensity, speed, or temperature of a device is needed.
