A crucial component of the patent application was a means of converting sound waves to electricity by causing the sound pressure to vary the resistance of an electrical conductor (a method still used today in carbon microphones).
Details of this were written into the margin of the application almost as an afterthought. Accusations flew thick and fast that Bell had been, illegally, given access to Gray's caveat, which included details of the technique and had amended his patent after submission.
According to the AT&T sponsored history. "Telephony The First Hundred Years" written by John Brooks and published in 1975, the patent "became the subject of thousands of pages of testimony in hundreds of suits to annul it, all unsuccessful...and it would be called, with few challengers, the most valuable patent ever issued."
A decade after the initial round of legal challenges to the patent had all failed Zenas F Wilbur, the patent examiner who had handled both Bell's and Gray's filings testified to a Congressional Enquiry that he had illegally notified Bell's attorneys when he had discovered a conflict between the two filings.