In this Instructable we will learn how to connect two 9 volt batteries to get:

+9V
0V
-9V

Simple 5 minute newbie special.

FYI: Why is negative voltage necessary anyway?

Negative voltage is not all bad. Yes, if you switch the orientation of a voltage source to negative polarity for many circuits, the circuit may not work anymore, but in other circuits that require negative voltage it has extreme use and the circuit cannot work without it.

One example of an electronic device that many times uses negative voltage is a transistor. Dual op amps need both positive and negative voltage to work.

*Note 0V is not the same as -9V. 0V is considered ground any other voltage is positive or negative.

I wonder if that's where they get the phrase "Let's start from ground zero." :)

Step 1: Parts

Two 9V Batteries
Alligator Clamps
Multimeter

Step 2: Battery Connection

Take two 9V batteries and stand them side by side. One positive and one negative will be connected in series.
Connect the middle terminals using an alligator clamp.
For the two remaining terminals, connect an alligator clamp. One for each side.

Step 3: Measure Voltage

With your multimeter check the voltage of the left battery by placing the black test lead on the ground between the two batteries and the red test lead on the alligator clamp on the left as pictured.

Now check the voltage of the battery on the right by placing the red test lead on the alligator clamp coming from the positive terminal.

Notes:

If a battery has positive orientation, its voltage with respect to ground is more positive than ground, and it delivers current to the positive part of a circuit. If a battery has negative orientation, its voltage with respect to ground is more negative than ground, and it delivers positive current to the ground part of a circuit.