Bell's patent and Gray's caveat were declared in interference with each other, a formal proceeding at the patent office in which the examiner has to determine whether, in the light of the interference, a patent should be granted to either party.
In fact, the two documents were very different. Gray's caveat covered a single method for transmitting speech. Bell's patent focused on a form of current that could be used in speech or telegraphy. Gray's patent heuristic was to cover speaking and harmonic telegraphy by patenting as many variations as possible; Bell's heuristic was to try to claim the whole landscape in a single patent.
The interference was resolved in favor of Bell's patent because it had arrived in the Patent Office a few hours earlier than Gray's caveat, though technically Gray still had three months in which to submit a patent and could also have contested Bell's claim in court. Gray's backers felt the speaking telegraph was a 'toy' that might be of occasional use over private lines, but would play no significant role in the transmission of multiple messages over long distances (Taylor, Unpublished Manuscript). Therefore, Gray did not contest Bell's patent until the commercial potential of the telephone became apparent.
After the interference had been voided, Bell learned from the patent examiner that the critical point of contention concerned a clause Bell had inserted at the last minute, in which he claimed the possibility of using variable resistance to create an undulating current. Gray's liquid transmitter depended on variable resistance. This conversation with the patent examiner became the source of endless debate during the years of litigation that followed, with some accusing Bell of outright theft of Gray's idea, in part because he eventually achieved the first transmission of speech with a device that looked similar to Gray's (Taylor, Unpublished Manuscript).
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