A Matter of Principle
The telephone had several inventors, all of whom built upon the innovations of their predecessors. Bell's patent No. 174,465 both credited him with the invention of the telephone and created a controversy that continues to provoke historians, scientists, and scholars. This controversy centers on the fact that, on the very day Bell filed his patent application, a caveat for a similar invention was filed by Elisha Gray. The caveat is no longer used today, but at the time it was a preliminary document that would have been filed to describe an invention that would eventually be the subject of a formal patent application.
The key similarity between Bell's patent and Gray's is that each describes a "principle of variable resistance," and details a liquid contact transmitter. Bell used such a liquid transmitter to demonstrate his invention at the 1876 exposition in Philadelphia, held in honor of America's centennial year. He also describes both the transmitter and the "principle of varying resistance" in his patent. This transmitter consists of a diaphragm, a needle, and a small cup of water. The cup of water is able to conduct electricity with the addition of a little acid. Speech is projected on the diaphragm, causing the diaphragm to vibrate. The attached needle is thus caused to vibrate in accordance with the speech. The vibrating action causes the needle to dip in and out of the cut of water, thus varying the resistance of the battery circuit. This variation creates the undulating current necessary for the electrical transmission of articulate speech.
The most serious charge against Bell was leveled after his patent officer admitted to having shown Bell the caveat submitted by Elisha Gray. This caveat described the "principle of variable resistance," which Bell had yet to develop on his own. The evidence suggests that Bell was able to incorporate Gray's principle into his own patent application before filing it: the variable resistance claim is written in on the margin of Bell's original patent application. Though the courts did sustain Bell's claims and named him the rightful inventor of the telephone, the evidence remains and the controversy lives on.
The electromagnetic receiver described by Bell in patent No. 174,465 is essentially the same as the telephone receivers in use today, and this feature is unique to his patent. Bell was accused of stealing Gray's "principle of variable resistance," a principle that was vital to the development of later electrical transmitters of speech. Though liquid is not used in current receivers, the "principle of variable resistance" played (and continues to play) a key role in the success of the telephone.