Adding a resistor to a series circuit will increase the total resistance of the circuit. This means that the current flowing through the circuit will decrease, according to Ohm's law (I = V/R). The voltage across each component in the circuit will also decrease, as it is shared among the resistors. If the voltage source is kept constant, adding a resistor will therefore reduce the voltage drop across other components in the circuit. This can have an effect on the overall performance of the circuit, such as reducing the brightness of an LED or decreasing the speed of a motor.
