Elisha Gray had reinvented the variable resistance…

Elisha Gray had reinvented the variable resistance telephone, but Alexander Graham Bell had been the first to write down the idea and the first to test it in a telephone.

Bell had developed an acoustic telegraph and had drawn up a patent application for it in 1875.

Since he had agreed to share U.S. profits with his investors Gardiner Hubbard and Thomas Sanders, Bell had requested that an associate in Ontario, George Brown, attempt to patent it in Britain, instructing his lawyers to apply for a patent in the U.S. only after they had received word from Britain (Britain will issue patents only for discoveries not previously patented elsewhere).

Meanwhile, Gray was also experimenting with acoustic telegraphy and thought of a way to transmit speech using a water transmitter.

On February 14, 1876, Gray had filed a caveat with the U.S. Patent Office for a telephone design that used a water transmitter.

That same morning, Bell's lawyer had filed Bell's application with the patent office.

There is considerable debate about who arrived first and Gray will later challenge the primacy of Bell's patent.

Bell, in Boston on February 14, had not arrived in Washington until February 26.

Bell's patent 174,465, is issued to Bell on March 7, 1876, by the U.S. Patent Office.

Bell's patent covers "the method of, and apparatus for, transmitting vocal or other sounds telegraphically ... by causing electrical undulations, similar in form to the vibrations of the air accompanying the said vocal or other sound".

Bell returns to Boston the same day and the next day resumes work, drawing in his notebook a diagram similar to that in Gray's patent caveat.

On March 10, 1876, three days after his patent is issued, Bell succeeds in getting his telephone to work, using a liquid transmitter similar to Gray's design.

Vibration of the diaphragm causes a needle to vibrate in the water, varying the electrical resistance in the circuit.

When Bell speaks the famous sentence "Mr. Watson—Come here—I want to see you" into the liquid transmitter, Watson, listening at the receiving end in an adjoining room, hears the words clearly.

Although Bell is accused of stealing the telephone from Gray, Bell had used Gray's water transmitter design only after Bell's patent was granted and only as a proof of concept scientific experiment to prove to his own satisfaction that intelligible "articulate speech" (Bell's words) could be electrically transmitted.

After March 1876, Bell will focus on improving the electromagnetic telephone and will never use Gray's liquid transmitter in public demonstrations or commercial use.

The question of priority for the variable resistance feature of the telephone had been raised by the Examiner before he approved Bell's patent application.

He told Bell that his claim for the variable resistance feature was also described in Gray's caveat.

Bell pointed to a variable resistance device in Bell's previous application in which Bell described a cup of mercury, not water.

Bell had filed the mercury application at the patent office a year earlier on February 25, 1875, long before Elisha Gray described the water device.

In addition, Gray had abandoned his caveat, and because Gray did not contest Bell's priority, the Examiner approves Bell's patent on March 3, 1876.