English: Liquid telephone transmitter (microphone) invented by US scientist Alexander Graham Bell around 1876. One of the earliest experimental microphones, Bell wired it to a battery and a crude earphone device to make experimental telephones. It was one of the first microphone devices that could transmit intelligible speech. Competing scientist Elisha Grey had submitted a patent claim for a similar liquid transmitter February 14, 1876, almost the same time that Bell was inventing his device, and there is some evidence that Bell may have stolen the idea from Gray. It quickly became obsolete and was not used in any practical telephones.

A fine wire extends down from the middle of a parchment diaphragm stretched over the bottom of the black mouthpiece (top). The end of the wire dips into the surface of water in the small brass cup under the mouthpiece. The water has been made conductive by the addition of a small amount of acid. When someone speaks into the mouthpiece, the diaphragm vibrates up and down, causing a greater or lesser length of wire to make contact with the water, changing the resistance. The binding posts at bottom are connected to the brass cup and wire, and a current from a battery is passed through them. The vibration of the needle causes the resistance to vary, causing a varying current to pass through the circuit. A speaker connected in the battery circuit responds to the varying current by reproducing the original sounds. Information from Liquid transmitter, Antique Telephone History]