This peculiar coincidence of simultaneous but separate work not only comes to light on the bringing out of great and important discoveries or inventions, but becomes more apparent if a new art is disclosed, for then the imagination of previous experimenters is stimulated through wide dissemination of the tidings, sometimes resulting in more or less effort to enter the newly opened field with devices or methods that resemble closely the original and fundamental ones in principle and application. In this and other ways there arises constantly in the United States Patent Office a large number of contested cases, called "Interferences," where applications for patents covering the invention of a similar device have been independently filed by two or even more persons. In such cases only one patent can be issued, and that to the inventor who on the taking of testimony shows priority in date of invention.[20]


[20] A most remarkable instance of contemporaneous invention and without a parallel in the annals of the United States Patent Office, occurred when, on the same day, February 15, 1876, two separate descriptions were filed in that office, one a complete application and the other a caveat, but each covering an invention for "transmitting vocal sounds telegraphically." The application was made by Alexander Graham Bell, of Salem, Massachusetts, and the caveat by Elisha Gray, of Chicago, Illinois. On examination of the two papers it was found that both of them covered practically the same ground, hence, as only one patent could be granted, it became necessary to ascertain the precise hour at which the documents were respectively filed, and put the parties in interference. This was done, with the result that the patent was ultimately awarded to Bell.