I recently bought a 12,000 BTU window air conditioner.  I live in an old house that only has two-prong outlets.  Of course the air conditioner has the three-prong (w/grounding) cord.  Is it safe to buy one of those three to two prong adapters or should I have an electrician put in a new three-prong outlet near the window? If I buy an adapter, I will still need an extension cord to reach the existing outlet.  What is the purpose of the grounding prong - reduce risk of shock, reduce fire hazard??

Install a new grounded outlet.

Do NOT use an extention cord.

I know not to use a standard $2 extension cord, but what about the heavier guage ones?  Aren't there some that are rated for appliances?

[b]What is the purpose of the grounding prong - reduce risk of shock, reduce fire hazard??[/b]

Yeah, but it will work with an adapter (if there aren't too many other things on the circuit). Use a heavy duty extension cord and maybe a some kind of overload breaker device would be a good idea.

Rewiring is, of course, the best solution.

I'll probably end up calling an electrician to put in a new outlet on a separate circuit.  I surely don't want to cause a hazard, but I thought I would run it by you guys to see if this was necessary.

[b]I'll probably end up calling an electrician[/b]

That would be best; I'm speaking as someone that has lived in an "old" house. 12,000 btu is kind of heavy for one of those old circuits.

Right - I did see the metal flange on the adapter that is supposed to be grounded to the screw in the face plate of the outlet.  The package says that this is only effective if the outlet's screw is grounded properly, which I have no clue how to tell.

It isn't.

That thing has a ONE horsepower compressor motor and a 1/4  or so for the fan.  Start current is pretty high.  You GOT to have a separate circuit.  #12 / 20 amp advised.

The third wire safety ground is for your protection.  When compressors and fans fail (as  well as other components) they often energize the case of the unit.  If you touch it, your lights can be the ones that go out.  This can happen and the unit will still work!  It is also possible to get enough leakage current to set the house on fire.  An overloaded circuit can do the same thing.  Either way, you need a new place to store guns!!