A variable resistor or, as it is also called, a rheostat is intended to adjust the resistance of the electrical circuit in order to obtain the desired value. Depending on its structural features, it can change the resistance both smoothly and in a stepwise order.
Usually a variable resistor is used in various types of equipment and devices. It can be seen as a volume control knob, as a dimmer, etc. Moreover, the scope of its application is so large that at present there is practically no electric device that, one way or another, would not be equipped with a variable resistor.
There are four main types of rheostats, which differ from each other structurally, but perform the same functions.
The first type is wire resistors. They are very common and can clearly show the principle of operation of all variable resistors. The essence of their work is that voltage is applied through the wire wound around the frame, and with the help of the third contact it is removed from various places of the frame. Naturally, in each particular place the resistance will be different.
The second type is a variable slide resistor. It is made of wire, which has a large intrinsic resistance, it is regulated in the same way as in wire resistors. However, there are differences. In the wire resistor, the voltage is removed from the contacts, which provides a stepwise change in resistance, and in the slide resistor, such a change occurs smoothly, since the slider freely slides along the wire.
The third type is a variable resistor that uses electrolyte. To do this, the resistor contacts are immersed in it, and adjustment is made by changing their distance.
One of the most obvious and simple types of variable resistors is tube. It consists of parallel-connected lamps, and the resistance changes in it by turning them off or on. Such a variable resistor is very inaccurate and unreliable, since its values directly depend on the degree of lamp heating, which means that they can change with time.
It is worth noting that, thanks to the rheostat, you can change not only the resistance, but also the current strength and voltage. For this, a variable resistor is included in the electric circuit in parallel or in series, depending on the need to change a specific parameter.
Thus, the variable resistors used to control the current strength can be called rheostats, and the resistors for voltage regulation are called potentiometers. It is them that are used as control devices for sound volume, output voltage, power, etc. A large number of different types of linear and angular displacement sensors were built on their basis.
Due to its qualities and properties, variable resistors are considered one of the foundations of modern electronics and electrical engineering.