A resistor is an ohmic device that limits the flow of charge in a circuit. Most circuits have more than one resistor. If several resistors are connected together and connected to a battery, the current supplied by the battery depends on the equivalent resistance of the circuit. The equivalent resistance of a combination of resistors depends on both their individual values and how they are connected. The simplest combination of resistors is the series combination.

In a series circuit, the output current of the first resistor flows into the input of the second resistor; therefore, the current is the same in each resistor. The potential drop across the resistor is equal to the loss of electric potential energy as current travels through it. According to Ohm's law, the potential drop V across a resistor when a current flows through it is calculated using the equation V = IR, where I is the current in amperes and R is the resistance in ohms.

The equivalent resistance of a set of resistors in a series connection is equal to the algebraic sum of the individual resistances. Any number of resistors can be connected in series. If n resistors are connected in series, the equivalent resistance is:

The disadvantage in a series circuit is that if something happens to one component, it affects all the other components. For example, if several lamps are connected in series and one bulb burns out, all the other lamps go dark.