# Illustration Series connection of three resistors and their current and voltage

Three electrical resistances $$R_1$$, $$R_2$$ and $$R_3$$ are connected in series, that means they are connected in such a way that the same current $$I$$ flows through all three. This characterizes a series connection of resistors!
For the current to flow at all, a voltage source is connected to the resistors, which supplies the voltage $$U$$. The positive pole of the voltage source is connected to one end of the 1st resistor and the negative pole is connected to one end of the 3rd resistor.
When resistors are connected in series, the individual resistors add up:$R ~=~ R_1 ~+~ R_2 ~+~ R_3$where $$R$$ is the total resistance of the series circuit. This is also called equivalent resistance.