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In this Illustration you can close and open switches to see what happens to the two identical bulbs. The table gives the voltage across the lightbulbs and the brightness of the bulbs indicates the relative current flow (voltage is given in volts). The batteries are all identical. Restart.

Notice that S1 and S2 cannot be closed or opened at the same time. What would happen if both S1 and S2 were open? What if both S1 and S2 were closed? (Which situation is bad for battery B1, and why?) If S1 and S2 are both open, no current flows, but if S1 and S2 are both closed, this creates a short circuit. There is essentially no resistance in the path connecting the two terminals of the battery, so as much current as possible flows from the battery (quickly discharging it).

As you switch S1 and S2 together (one open and one closed), what happens to the lightbulbs? Why? (That is, what is the difference between the two circuits?) Notice that when you put another battery in series, the potential difference (voltage) across each bulb increases, as does the current (and the bulbs are brighter).

With S1 closed and S2 open, now close S3. What happens? What is the voltage across each bulb? Why doesn't it change? You have added a battery to the circuit, but nothing happened. Why? Now close S1 and open S2. What happens? Why?

Now look at the voltage across bulb B. When S1 is closed, S2 is open, and S3 is open, the voltage across bulb B is 9 V. When you close S3, the voltage across bulb B is still 9 V. With S3 closed, the voltage across bulb B is always 9 V, no matter what is happening with S1 and S2. When you put batteries in parallel, you don't increase the total voltage. In fact, you need to be careful of adding batteries in parallel because if you have two batteries in parallel with each other at different voltages, you will end up with a great deal of current. This is why you have to be careful using jumper cables: You put two batteries in parallel with each other and even the slightest difference in voltage means a lot of current flows (because the jumper cables don't have much resistance. The current is limited by the internal resistance of the batteries). Of course this also works to your advantage to quickly recharge the dead battery.

Illustration authored by Anne J. Cox.