The very first liquid transmitter was designed by Elisha Gray, an American electrical engineer. Until February 1876, he kept his invention secret. A few days before submitting his application to the Patent Office, information about the transmitter became known to Bell. By bribing a Patent Office employee, Bell got his hands on some drawings and specifications relating to Gray's invention. Bell hastily prepared the necessary documents and, without even a prototype transmitter, returned to the patent office with his application. Interestingly, it was registered as having been filed earlier than Gray's. Gray's patent application contained more accurate and detailed information on the invention in comparison to Bell's. Until his death, Gray believed that he had literally 'invented' the telephone for Bell. Having already received his patent, Bell then went about making his liquid transmitter (based on Gray's drawings and descriptions) and, together with his assistant Thomas Watson, conducted the now famous sound experiments. According to popular legend, on 10th March 1876, Bell was on the same floor with the transmitter with Watson upstairs with a receiver. During the experiments, Bell accidentally spilled acid on his trousers and shouted the now historic words: "Mr. Watson, come here! I need you!" Watson heard Bell's words coming from the receiver. Bell had no idea that the device he was working on could actually transmit the human voice over a distance.
The principle of operation of a liquid transmitter is simple enough. Water is poured into the cylindrical reservoir. A membrane diaphragm was stretched across it's width. A small length of wire was attached to this 'diaphragm' so that it extended downwards making miniscule contact with the water. A small amount of acid was added to the water to enhance its electrical conductivity. When the human voice was projected into the 'funnel' shaped mouth-piece, the diaphragm vibrated causing the length of wire in contact with the water to change. Thus the electrical resistance of the circuit changed. With high-quality assembly the liquid transmitter can be very sensitive but, unfortunately, the transmitter can not be moved without liquid spillage (therefore causing signal interruption). In addition, the water itself would eventually evaporate. Bell's Liquid Transmitter was soon replaced by the electromagnetic system developed by Thomas Watson, in which he used a funnel, a metal diaphragm and a permanent magnet. Bell used this sort of telephone design at his first major public demonstration at the official Centennial World's Fair in Philadelphia, PA on 25th June 1876. This aroused huge interest, not least in such notable dignitaries as the Emperor of Brazil Dom Pedro II, Lord Kelvin and other important people. In 1885 the American author and journalist Samuel Adams Drake wrote in his book 'Our Great Benefactors': "Had the Sphinx opened its granite lips, surprise could hardly have been greater or more genuine…. Indeed, it was one of those amazing discoveries that, had it taken place in the days of the Inquisition, the inventor would have been accused of witchcraft."