The Telephone Cases, 126 U.S. 1 (1887) (full-text).
U.S. Supreme Court Proceedings Edit
The Court found that an apparatus claim for the electrical transmission of speech was patentable because there exists an equivalence of the changes in substances (the Cochrane test) and electromagnetic alterations (“electrical undulations”) for subject matter purposes. The Court said that "electricity, one of the forces of nature, is employed; but electricity, left to itself, will not do what is wanted. The [invention] consists in controlling the force as to make it accomplish the purpose” of transmitting messages across long distances.