Many researchers think that all searching can be done electronically – and this is essentially true for U.S. patents granted since 1976. Patent searchers, especially inventors who need to thoroughly search the entire realm of patents to ensure their idea hasn't already been patented, have more limited options available electronically and for free. Older U.S. patents do not have as many online search options because the patent pages were put into the USPTO database as scanned images, and older patents from outside the U.S. can be even more challenging to find.
The patent number is the magic key to the patent information system. Because patent numbers are often found on manufactured objects, collectors often use patent numbers to find information relating to a particular antique object. Regardless of what date the patent was issued, if you know the patent number, you can quickly pull up the full-text patent by searching Google Patents (using the box to the left), the US Patent & Trademark Office site, or the pat2pdf website. Almost all free sites will allow you to enter a U.S. patent number and retrieve a PDF version of the patent.
You can also search for patents by topic or inventor, but options vary depending on how far back you want to search:
Search options are more complicated, but improving for older U.S. patents. Many early patents are now full-text searchable through Google Patents, although the digital texts were automatically generated and may contain gibberish or be difficult to search.
U.S. patents issued from 1790 through 1975 may only be searched on the USPTO website by Issue Date, Patent Number, and Current US Classification. Google Patents Advanced Search screen will allow you to search by patent number, inventor, assignee, topic, classification number, and date. Once you have searched, you can also refine your results by issue date, publication date, patent office, filing status, and patent type.
Search either Google Patents or Espacenet by keyword (see the Free Patent Websites tab on this guide for more details). Google Patents will allow you to specify the field you want it to search in but only if you use the Advanced Search.
If you do not have a specific patent in mind and just want to search for U.S. patents by subject there are several different options: