The year was 1876 when Alexander Graham Bell created and tested the very first telephone, a device that was to change the future of communications for ever. Bell named this first telephone "Liquid Transmitter". A brass rod is emerged into a conductive liquid. In the same liquid there is also a wire which is suspended by a membrane. Sound waves heating the membrane cause it to vibrate. These vibrations are transferred to the suspended wire. As the wire vibrates inside the conductive liquid, the resistance between the wire and the brass rod changes accordingly. This change in resistance results in current change.
The very first test was done by Alexander Graham Bell and Mr. Watson in his house. Being in separated rooms with all doors closed, Bell asked Mr. Watson:
Mr. Watson - come here - I want to see you
This is a very interesting video uploaded by ScienceOnline which explains precisely how the original Liquid Transmitter works. Following the original drawings and notes from Bell, he also creates a working replica of the Liquid Transmitter. You can follow this step-by-step guide to re-create your own working historical replica!