A resistor that has the property of increasing in resistance as current flowing through it increases, and decreasing in resistance as current decreases. Therefore the ballast resistor tends to maintain a constant current flowing through it, despite variations in applied voltage or changes in the rest of the circuit. The ballast resistor acts as a variable load on the system; therefore it differs from load resistors, which have a constant resistance. Ballast resistors are now mainly of historical interest; electronic devices fulfilling the same function have replaced them.