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Most companies already have a backlog of ideas of their own awaiting development and don't want any more.
Most companies fear legal entanglement with outside inventors. Many refuse to look at outside inventions unless they are covered by issued patents and confine their consideration to the allowed claims in those patents. Some have separate departments to respond to inventors. Some send a discouraging list of conditions under which they will even look at outside inventions.
Some may actually use the invention and defy the inventor to sue for infringement of his patent.
A number of companies offer to sell your invention for you, for a fee. I have never heard of a success. Before giving money to such a company, see the following web sites suggested in the November 2003 issue of the IEEE magazine, Spectrum:
http://www.uspto.gov/web/offices/co/iip/data/.htm#LicensePromotion
Kamm's Law: "Most people are hostile to most new ideas and are at their most creative when inventing objections to them."
All of this is extremely frustrating, but unfortunately, true.
However, hope springs eternal, particularly in inventors, so read on.
A few inventors achieve success by stopping with a patent and then waiting to sue an infringer.
Some companies start with the inventor's own money and bootstrap with early results to recruit further investment. I have done so, but it was a near thing. VERY RISKY. DO NOT USE YOUR HOUSE AS COLLATERAL FOR A LOAN. There is a long line of inventors with lost houses.
You might do well to pay a consultant a small sum to give an opinion of your invention before you invest serious money in it. Expect to pay him a retainer (advance payment) since he does not know you, and expect to receive a confidential disclosure agreement after you have paid the retainer and before you disclose your invention.
Investment capital can come from your own savings, bank loans, SBA loans, family, friends, venture capital companies, private corporation stock, and public corporation stock. Read up on company finance.
The slow but sure road to moderate success is to get a job with a company which values internal inventions. There lies praise, promotions, and raises. There also lies the path to spinoff using the employment experience as training. I have done this.
Get an MBA to learn about business.
Be aware that the Patent Office has an elaborate program to aid independent inventors. Use their email address independentinventor@uspto.gov to ask questions.
Read the Patent Office publications, starting with "General Information Concerning Patents" and other books on patents (Amazon has many.).
Do as much as you can by yourself, particularly do a patent search and file either a "Disclosure Document" or a "Provisional Patent Application" for small fees. Both are described in the references below.
(A Provisional is a cheap do-it-yourself informal disclosure that establishes an early filing date if a full application is filed within a year. It's only benefit is to win an interference with a patent application filed by a thief who learns the invention and pretends to be the inventor. It give some assurance to the inventor who can then disclose his invention to sell or license it. If that effort fails, the inventor can abandon his effort without the expense of a full search and application. I have done this several times.)
Go to a patent attorney or patent agent (they are equivalent) only when you are ready to invest several thousand dollars to generate a patent.
General Information Concerning Patents, Superintendant of Documents Stock Number: 003-004-00683-8 ($3.75)