: Suppose somebody invents (say) the antigravity motor and patents it.
: Could I then jump on board with a patent which specified its use in
: (say) aircraft and other 'obvious' applications?
First, I'll assume that your definition of obvious is different
from the legal definition. The legal definition of obvious is that it is
apparent to anyone skilled in the art. I'll assume that your definition
of obvious means something more like that it seems like a natural
extension of the original patent. The difference between these two is
very murky.
For the example that you give, the answer is most likely yes, that
you could obtain a patent, but that it may or may not be of much use to
you. I'll illustrate why by providing an example that one of my former
patent attorneys uses to describe what a patent really legally means.
A patent is a "negetive right" obtaining a patent does not give
you the right to do anything, per se, but it takes away the right of
others to profit from your invention. Use a pencil as an example.
Suppose someone invented pencils and received a patent on that invention.
Suppose you realize that you can color the lead of a pencil, and invent
red pencils. Most likely, this new invention would not be deemed obvious
according to the legal definition. You receive a patent for your red
pencil. Who can sell red pencils? The answer is no one. The inventor of
pencils holds the patent for pencils (a red pencil is a pencil) so you
could not profit from the sale of your red pencils without infringing upon
the original patent on pencils. Conversely, even though a red pencil is a
pencil, it is a unique invention, and the patent holder for pencils could
not profit from its sale without infringing. All this assumes that no
licensing agreements exist, of course.
So, that example is, I believe, very analogous to yours. You
would, most likely, be able to patent an airplane with an antigravity
generator, but your couldn't sell them without infringing upon the the
patent for the antigravity generator (without a licensing agreement, of
course.)
Hope that helps,
Joe