Journal articles and other periodical publications provide scholarly information on specific aspects of history and the history of science. Since articles are generally published on a shorter schedule than books, they can be more-up-to-date. The following indexes are the best ways for you to identify journal articles:
There are also indexes devoted to other subject areas that may be of use for specific topics:
Usually researchers find more sources by looking at the footnotes in an article or book, but these will always be older than the publication you have in hand. Citation indexes like the Web of Science (which includes sections for the Social Sciences, Arts and Humanities, and Science) are set up to search for sources cited in the footnotes of journal articles as soon as they become available.
This allows you to find newer articles which cite the books and articles you already know are key for your topic. By relying on connections between authors rather than subject words and by moving forward in time, citation searching can open up new avenues of research.
See this tutorial for more information on cited reference searching.