In a series circuit the total resistance is the sum of all of the resistances of the individual components making up the circuit (Compare >> parallel << circuits.)



The way to see this is as follows:



From the diagram above you can see that the potential difference is that of the cell, i.e. 12 V. The resistors are (deliberately) identical at 3 Ω  (ohms) and the current across the circuit is 2 A.


As the current is constant around the circuit, the current passing through each of the resistors is 2 A. The potential difference across each resistor is given as 6 V but can be proven using the simple formula V = IR:




Likewise the total resistance of the circuit can easily be seen to be 6 ohms but can be calculated by simply rearranging V = IR and substituting the known values:



Any resistance that the ammeter might have is taken to be negligible and can be disregarded.