SERIES ELECTRIC CIRCUITS have elements arranged one after another along the circuit. The current therefore flows through each element in turn. If one element is removed then the circuit is broken. This animation shows a simple DC electrical circuit with one bulb. About halfway through the movie a second bulb appears along the wire. Notice how both bulbs become dimmer and how the amount of current flowing drops.
EXPLANATION: as the second bulb appears so the resistance of the circuit goes up. In this simple case we ignore the resistance of the wire. Adding an identical second bulb will double the resistance of the circuit. The voltage provided by the battery (cell) stays the same. With double the resistance, the current drops by half. With half the current flowing down the same voltage drop, the energy dissipated by the entire circuit drops by half. Furthermore, this reduced amount of energy is now divided between the two bulbs. Consequently, when you add a second bulb the brightness of the bulbs is only a quarter of the brightness of the bulb in the original circuit.
Actually, of course, things are never that simple. The brightness also depends on the temperature of the filament, and that will drop as the current drops. However, it is reasonable to say that in the circuit with two bulbs in series, the energy released by each bulb is a quarter of that released by the single bulb in the original circuit.SI UNITS used in electricity: