http://allfreeprintable.com/cont/sgn/img/printable-danger-high-voltage-sign.jpg Voltage What is voltage? Voltage is the electrical potential energy per unit charge or the difference of charge between two points. Think of it as water pressure or PSI. The units for voltage are in joules per coulomb which simply are called volts. This electrical potential is the pushing force for all electrical circuits. Without it there would be no current and this can be confirmed using Ohm's Law. The arrangement of the loads or in this case the two lamps will determine how much voltage each lamp will get. In the prior circuit, the lamps were wired in parallel meaning both loads share common nodes. In the next diagram the loads will be wired in series, meaning there is only one path to get through both of them. Notice how the voltage of each lamp is only 6 volts. This confirmed using  Kirchhoff''s Voltage Law, KVL. This states that the sum of the voltages in a given loop (complete path) is zero. -V(source) + V(lamp1) + V(lamp2) = -12v + 6v + 6v = 0v If we add another lamp in series then each lamp will drop one third the voltage as long as each lamp is the same resistance. -V(source) + V(lamp1) + V(lamp2) + V(lamp3) = -12v + 4v + 4v +4v = 0v Each time a lamp is added, the brightness of all the lamps are decreased because the overall resistance of the circuit increases making the current decrease. This is similar to how Christmas lights are wired. The downside series type circuits is, if one lamp goes out they all go out. The total resistance is 3 + 3 + 3 = 9 Ohms. V / R = I resulting in 12 Volts / 9 Ohms =  1.33 Amps.