This circuit is an example of two light bulbs in series. When a circuit is in series, the current is exactly the same at all points along the wire because there is only one path in which the current is able to flow (which is why the bulbs are equally lit). The total resistance of a circuit in series can be measured by finding the sum of the individual resistances. The sum of the drops in voltage throughout the circuit equals the total drop in voltage for that circuit. If I were to remove one bulb from the circuit, the other light bulb would shine brighter because there is more voltage going to that individual bulb.
In this illustration, the circuit is constructed in a parallel format. One lightbulb is directly above the other one. In parallel circuits, each resistor results in a different path for the electrons to flow along. The total current in this kind of circuit is equal to the sum of the currents flowing through each resistor. The equivalent resistance in parallel circuits decrease when each new resistor is added. Finally, the voltage drop across each branch is equal to the source of the voltage (the battery). If I were to remove one light bulb from the circuit, the other light bulb would still shine because it has an individual connection to the battery.
In this example, the circuit is complex (also called a combination circuit). In complex circuits, the current is found by first finding the resistance in the two parallel circuits, and then adding that to the resistance of the light bulb that is in series. After determining the total resistance, the current can be determined. In this type of circuit, the voltage drop in the parallel portion is always equal, but the drop existing in the resistance in series is found seperately. The total current also varies from each resistance. In my example, the light bulb in series is glowing brighter because there is more current running through that portion of the circuit. If I were to remove the light bulb that is in series, the two bulbs that are in parallel would both shine brighter because the voltage that the bulb in series took up no longer exists. If I were to remove the top parallel light bulb, then the bottom parallel bulb would now be in series with the bulb that was not part of the parallel part of the circuit, and they would both shine. If I were to remove the bottom bulb that is in parallel, the top one in parellel would go out also because there is not a connection to the battery. The bulb that was in series would still shine (brighter) because it is directly connected to the battery - and is able to use all of the voltage that the battery provides.
