A simple op-amp circuit can be used to a split a supply into equal (or even unequal) parts by setting it up as a follower and biasing the non-inverting input to half supply with a high-impedance, because the op-amp is working on a voltage, so 100K + for your divider. The output from the op-amp is now effectively at 0v with respect to the two supply rails so you use that as your ground.

This is correct. The Wien-Bridge reduces the amplitude to 1/3, so you need a voltage gain of 3 to let the circuit resonate.

Hi, sorry I'm slow at answering comments. The circuit shown is from the book "Practical Electronics for Inventors" by Paul Scherz and Simon Monk on page 693 (p. 722 on a pdf viewer). It's a great resource, hope this was useful!

peatmoss4415, the basic Wien bridge oscillator (at least according to my old theory books, but the Wikipedia article confuses the issue) does not use an amplitude stabilizer element. But regardless of that, for this type of oscillator to work well it should have some kind of amplitude stabilizing element in the feedback path. Hewlett is supposed to have originated the idea of using a small incandescent lamp for this element, while other versions use inverse-parallel LEDs, etc. And the Wien bridge oscillator is not named after the German word for wine, nor is it named after the German name for the city of Vienna, rather it is named after its inventor Max Wien, which is pronounced 'veen'.

Yeah works well with a bulb.