Alexander Graham Bell’s big breakthrough came on 10 March 1876 when he used what he called a ‘liquid transmitter’. This was a vertical metal cone with a piece of parchment stretched like a drum over its narrow end at the base. On the outside of the parchment, Bell had glued a cork with a needle stuck in it, pointing into a tiny cup of diluted sulphuric acid.
When he shouted into the open end of the cone, his voice made the parchment vibrate, so the needle moved slightly in relation to a contact in the cup. The needle was wired to a battery and the movement varied the strength of the current passing between the contacts, thus converting sound waves into an electric signal which travelled along a wire to a receiver. While setting up the experiment, Bell spilt some acid on his trousers. Shouting to his assistant, Thomas Watson heard the message on the receiver in another room and rushed through to Bell, who had just made the first phone call.
Answered by Alison Taubman, BT Connected Earth curator, NMS