Google Inc. is developing a user-generated online encyclopedia that could rival Wikipedia by allowing users to write authoritatively on subjects they are knowledgeable about and allow Google to place ads on this site.

In an announcement on Google's corporate blog, Udi Manber, a vice president of engineering, said a knol on a particular subject is meant to be the first thing someone who searches for this topic for the first time will want to read. The site is currently being privately tested, the blog said, and no release date was confirmed.

Google has named the encyclopedia the knol project, a knol being a unit of knowledge. The company aims to tie strong identities to contributing authors and those seeking to edit knols.

Our goal is to encourage people who know a particular subject to write an authoritative article about it, wrote Manber. The goal is for knols to cover all topics, from scientific concepts to medical information, from geographical and historical to entertainment, from product information to how-to-fix-it instructions.

The popular search engine's primary focus on Knol will be with the authorship of each page. The original author of each page will have a miniature profile on the page. This will give them access to different options to control the page.

The main difference between Knol and existing websites like Wikipedia, is that authors will be given the option to place Google ads on the pages they write and receive revenue from those ads.

Wikipedia has a huge head start with more than 75,000 active contributors, 2.1 million English-language articles and a dominant position in search results. Google indicated it would rank knols in the same manner it ranks all Web pages, not giving itself a competitive edge over existing sites.