As I said, you have +12V and -12V.

 

....you do NOT have a 24V source....

What do you mean by this? I can get 24v. Am I wrong?

 

What do you mean by this? I can get 24v. Am I wrong?

You can get 24V if you measure between +12V and -12V. How you use 24V in your circuit is another thing. What you really have are two 12V supplies placed in series with the center tap grounded.

 

you got a lot of good replies already, but...

- you do not want to use instruments to conduct current, basically in your circuit, you have no path for current on your breadboardhas to go through grounded instruments to close the loop. I would actually say it has potential danger if something goes wrong in the grounding of your instruments.
- choose a rail for ground and connect the negative supply of your function generator to that rail, you will use that as you ground reference for all your measurements. When you are figuring out what voltages you have, you may have 24V in the circuit, but is it with respect to ground? This is where ground rail comes in very handy. As well as figuring out whether you have a chance of shortening something with your probes.

- there is also an issue of creating virtual grounds which comes up when working with op-amps, etc. but again the basic question is always what are you measuring with respect to? where is your reference point?

For virtual gound: you can take a 24V source, as measured from your ground reference of 0V and use a voltage divider to split it into +12V and -12V where your mid-point of the voltage divider becomes a virtual ground. But there is no way to mae a 24V source out of 12V and -12V.

That is a basic idea of what ground is.