In a nutshell
David Hughes made his fortune by inventing the Type Printing Telegraph and the first microphone. His trademarks were simple components and clear thinking.
Hughes' first invention was the Type Printing Telegraph, which was soon used everywhere by the Western Union Telegraph Company, USA. Hughes became famous throughout Europe. Fortune made, he went back to London to live a quiet life.
However, it wasn't long before his inventive streak struck again, this time to invent the microphone. He showed the new device to an audience of distinguished scifiles in London. David could have made a huge fortune if he'd patented the device, because he had a clear vision of its potential. But he chose, instead, to give it to the new world of telecommunications.
His microphone and other inventions are kept at the Science Museum in London. When he died, he left £400,000 (a huge sum in those days) to five hospitals in London and a bequest to the Royal Society.
A microphone converts sound waves in air into electricity in a circuit. When our vocal chords vibrate, they generate waves (high and low values) of air pressure. These changes of pressure cause compressions of different values on different materials - carbon, for example, in the body of the microphone. It's a peculiar characteristic of materials like this that their electrical resistance changes under pressure, and these changes can cause changes of electrical current in a circuit.
In a radio transmitter, the changes in current cause aerials to emit radio waves. At the receiving end, which might be on the other side of the world, the varying radio waves cause changes of current in the aerial which cause changes in materials in the device, making a diaphragm vibrate, generating sound waves. This last stage is called a loudspeaker. So, a microphone and a loudspeaker can be thought of as equals but opposites.