A
rheostat is an electrical component that has an adjustable
resistance. It is a type of
potentiometer that has two terminals instead of three. The two main types of
rheostat are the rotary and slider. The symbol for a
rheostat is a
resistor symbol with an arrow diagonally across it. They are used in many different applications, from light dimmers to the motor controllers in large industrial machines.
Rheostat Construction
Most
rheostats are the wire-wound type that have a long length of conductive wire coiled into a tight spiral. The linear type have a straight coil while the rotary type have the coil curved into a
torus to save space. The coil and contacts are sealed inside the case to protect them from dirt which can cause an open circuit, and from moisture which can cause a short circuit.
Rheostats can be made from other materials such as
carbon disks, metal ribbons, and even certain fluids. As long as a material has a significant
resistance change over a short length, it can probably be used to make a
rheostat.
How they Work
The basic principle used by
rheostats is
Ohm's law, which states that
current is inversely proportional to
resistance for a given
voltage. This means the
current decreases as the
resistance increases, or it increases as the
resistance decreases.
Current enters the
rheostat through one of its terminals, flows through the wire coil and contact, and exits through the other terminal.
Rheostats do not have
polarity and operate the same when the terminals are reversed. Three-terminal
potentiometers can be used as
rheostats by connecting the unused third terminal to the contact terminal.
See Also
Ohms Law
Potentiometer
Resistor