Direction of the current flows one path.
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Series Connection 

(1) A connection of two-terminal networks that causes a common current to be carried through the networks, since there is only a single path for the current. Series-connected electric power sources are used to obtain a voltage that exceeds the electromotive force of a single source. If power-consuming devices, or loads, are series-connected, the voltage across the loads is distributed proportionately to the resistances of the loads. Disconnecting one element interrupts the current flow in all parts of the circuit.

(2) A connection of four-terminal networks when the voltage and the current at the output of the preceding network are equal to the corresponding voltage and current at the input of the next network. Series-connected four-terminal networks are used to increase attenuation or amplification in signal conversion devices; they are also used in electric simulation of a combination of elements of automatic-control systems.

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