If you have an idea for an invention, before you patent it, you should check to see if it's already been invented. Whether or not it's been invented, though, you should find out whether it's been patented before with the U.S. Patent and Trademark Office (USPTO). Search patents in person, online or at your local library to find out if your invention idea is already patented or if you should redirect your brainstorming to another invention idea. When you conduct the online patent searches, you'll be able to read the full text of the patents and see diagrams.
Visit the U.S. Patent and Trademark Office if you live near Alexandria, Virginia. The USPTO facility has print archives of patents filed in the U.S. as far back as 1790.
Check at your local library for a collection for microfilm containing all previous patents filed. Ask for help with the Reference department. Some libraries are Patent and Trademark Depository Libraries and keep in their holdings in microfilm or optical disc format.
Visit the USPTO website, where you can search the database for patents that were granted by the Patent and Trademark Office since 1976, or do a Google Patent search, where you can use advanced search options limiting your results by such things as inventors and patent dates. See the Resources section for the links.
Be creative with your patent search terms. What you call your invention idea may not be what it was called when it was patented. Look for variations on your idea, as well as your specific idea, and check to see if there is a use (usage or utility) patent, also.